Tunable multimode laser diode module, tunable multimode wavelength division multiplex raman pump, and amplifier, and a system, method, and computer program product for controlling tunable multimode laser diodes, raman pumps, and raman amplifiers

ABSTRACT

A tunable multimode wavelength division multiplex Raman pump and amplifier, and a system, method, and computer program product for controlling a tunable Raman pump and amplifier. The tunability of the pump source is accomplished by either straining or heating an external fiber grating, thereby causing a different wavelength of light to be emitted by the pump source. The system includes a microprocessor-based controller that monitors an amplifier&#39;s performance and adjusts the drive current and/or wavelength of the tunable pumps of an amplifier to achieve a target performance.

CROSS-REFERENCE TO RELATED PATENT DOCUMENTS

This document contains subject matter related to that disclosed in U.S.Pat. No. 6,292,288 to Akasaka et al., filed on Mar. 17, 2000, entitled“RAMAN AMPLIFIER, OPTICAL REPEATER, AND RAMAN AMPLIFICATION METHOD”; andU.S. patent application Ser. No. 09/775,632, filed on Feb. 5, 2001,entitled “RAMAN AMPLIFIER SYSTEM, APPARATUS, AND METHOD FOR IDENTIFYING,OBTAINING, AND MAINTAINING AN ARBITRARY RAMAN AMPLIFICATION PERFORMANCE”(Attorney Docket No. 199455US-8) the entire contents of each of whichbeing incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tunable multimode wavelength divisionmultiplex Raman pump and amplifier, and a system, method, and computerprogram product for controlling the same.

2. Discussion of the Background

With the explosion of the information age has come a demand for largerdata transmission capacity for optical communication systems.Conventionally, optical communication systems transmitted data on asingle optical fiber using a single wavelength of light (e.g., 1310 nmor 1550 nm). Signals at these wavelengths were desirable since they havereduced light absorption properties for optical fibers. However, inorder to increase the data transmission capacity of these single fibersystems, it was necessary to increase the number of optical fibers laidon a transmission route which greatly increased the cost of opticalfiber networks.

To mitigate this problem, wavelength division multiplexing (WDM) opticalcommunications systems such as the dense wavelength divisionmultiplexing (DWDM) system have become desirable. In a WDM system, aplurality of optical signals, each having a different wavelength, can betransmitted simultaneously through a single optical fiber.

Optical fiber communication systems transmit optical signals overconsiderable distances. However, the signal strength of the opticalsignals attenuates with distance because of absorption and scattering.Signal strength attenuation ultimately results in signal receptiondegradation if the signal strength is not kept above background noise(or other sources of noise) by a predetermined amount. Amplifiers areused to keep the signal strength above background noise by apredetermined amount. In general, there are two approaches to amplifyingan optical signal: the first, is to use an electronic repeater, whichconverts the optical signal into an electric signal, amplifies theelectrical signal, and then converts the amplified electrical signalback into an optical signal for further transmission along an opticalfiber; the second, is to amplify the optical signal itself. Two types ofamplifiers that can be used to amplify an optical signal according tothe second approach are rare earth doped fiber amplifiers such as erbiumdoped fiber amplifiers (EDFA), and Raman amplifiers.

EDFAs are currently the most widely used optical amplifiers for WDMsystems and are effective and reliable for optically amplifying WDMsignals. However, EDFAs have an amplification bandwidth that is limitedin range, and produce a wavelength-dependent gain profile. These twocharacteristics of EDFAs are undesirable for WDM signals, which arespectrally distributed, since a non-uniform amount of gain will beapplied to the various WDM channels, depending on the wavelength of thechannels. To offset this effect, a gain flattening filter may be used toobtain a uniform or flat gain profile (having a gain deviation of lessthan 1 dB) across the entire communication band. The gain flatteningfilter is designed to have a loss profile having a shape that is theinverse of the shape of the gain profile. Gain flattening filters,however, are limited to a particular gain profile, and are notdynamically adjustable to compensate for changes in a magnitude of thegain of the EDFA. Therefore, a flat gain profile cannot be maintainedwhen the gain of the EDFA is changed, or if the attributes of thecommunications network are changed, such as by adding more WDM signals.In addition, the gain flattening filter decreases the total amount ofpower launched into an optical fiber.

Raman amplifiers use a phenomenon known as Stimulated Raman Scattering(SRS) of light within an optical fiber to achieve a gain in a particularwavelength band. The inelastic scattering process generates an opticalphonon and a co-propagating Stokes wave, light that is downshifted infrequency from the pump light by an amount equal to the phonon frequency(i.e. total energy is conserved). In silica fibers, the peak SRS gainoccurs at about 13 THz below the pump light frequency (or conversely, ata wavelength that is longer than a wavelength of the light pumped intothe optical fiber by about 100 nm). Since Raman amplification is ascattering process, unassociated with the resonance properties of anyparticular material, one can generate a Raman gain spectrum for pumplight at any wavelength. Therefore, changing a wavelength of the pumplight, changes the wavelength at which a peak gain is applied to WDMsignals, thereby amplifying some WDM signals more than others. Bymultiplexing several different pump wavelengths into the same fiber, onecan generate a reasonably flat gain spectrum over an arbitrarybandwidth. Because Raman amplifiers require a greater pumping power toobtain the same gain as an EDFA, Raman amplifiers have primarily beenused in signal wavelength bands outside of the amplification bandwidthof EDFAs.

Although a Raman amplifier amplifies a signal over a wide wavelengthband, the gain of a Raman amplifier is relatively small and, therefore,it is preferable to use a high output laser device as a pumping source.However, increasing the output power of a single mode (or frequency)pumping source beyond a certain threshold leads to undesirablestimulated Brillouin scattering and increased noise at high peak powervalues. As recognized by the present inventors, to prevent this problem,a multimode laser device is preferably used as a pumping source in aRaman amplifier. A multimode laser has a plurality of oscillatinglongitudinal modes, each providing output power at less than thethreshold at which stimulated Brillouin occurs. A multimode laser canprovide a sufficient amount of output power to achieve Ramanamplification distributed over the various modes (i.e., wavelengths ofoutput light), as opposed to providing the power all at a singlewavelength.

To control the wavelength of the light emitted from the pumping source,and therefore, determine what wavelength of signal will be amplified, itis well known to use fiber gratings. A fiber grating selectivelyreflects certain wavelengths of light causing a laser beam of a specificwavelength to be output. Fiber gratings are known to be included in thecore of an optical fiber, separate from the laser device itself.

Spectroscopy is one application for using a tunable fiber grating isdescribed in U.S. Pat. No. 6,188,705, the entire contents of which beingincorporated herein by reference. A tunable fiber grating is describedas being coupled to a quasi-monochromatic light source for selecting asingle frequency of light output from the light source. Although thelight source is capable of producing multiple modes of light, thetunable fiber grating is configured so that only a single frequencyoutput of the light source results. As explained in U.S. Pat. No.6,188,705, for spectroscopy applications, signal frequency operation ishighly desirable, if not required, in order to detect substances withnarrow bandwidth absorption lines ('705 patent, col. 1, lines 54-57). Awavelength tuning mechanism may tune the fiber grating by way oftemperature change, compression, or through the application of stress ofstrain ('705 patent, col. 6, lines 14-42).

The present inventors have recognized, however, that the singlefrequency aspect of the tunable fiber grating described in the '705patent would render it unsuitable for use as a Raman amplifier pumpdevice because, as discussed above, multimode pump sources are superiorto single frequency pump sources when used in Raman applications.

As described in Bruce, E. “Tunable Lasers,” IEEE Spectrum, pages 35-39,February 2002, there are a variety of other types of tunable lasers madefor use in WDM systems, although the primary application is forgenerating a WDM signal at a particular frequency (or wavelength). Asrecognized by the present inventors, since none of these tunable lasersare made for operation as a Raman pump source that intentionally outputslight at more than one frequency, it is unclear from the literature how,or whether, these devices could be adapted for use in multimodeapplications.

As described in U.S. Pat. No. 6,292,288, in order to achieve a uniformgain profile over a broad range of wavelengths, a Raman amplifier caninclude multiple pump lasers, each providing multimode light having apredetermined spectral width, centered at a different centralwavelength. By properly spacing in wavelength the pump lasers withpredetermined optical output levels, it is possible to achieve acomposite gain profile that is flat over a broad range of wavelengths,and therefore to provide Raman amplification over a broad range ofwavelengths.

U.S. patent application Ser. No. 09/775,632 describes a system throughwhich Raman amplification performance can be controlled. As described inthat application, by controlling an output power of each of the lasersof a particular Raman amplifier, the desired gain characteristics can bemaintained. Moreover, by monitoring and controlling a portion of anetwork, the Raman amplification performance of that portion can becontrolled through cooperative adjustments made to one or more of theRaman amplifiers (or of individual pump lasers of a particular Ramanamplifier) that impact that portion of the network.

FIG. 1 is a block diagram of a conventional Raman amplifier 100. TheRaman amplifier 100 includes an amplifier fiber (optical fiber) 103, aWDM coupler 104, a pumping device 107, a control unit 119, and optionalpolarization independent isolators 102, 105. The Raman amplifier 100 isconnected (or merely coupled) to an input fiber 101 and an output fiber106, which may be optical transmission fibers such as single mode fibers(SMF), dispersion compensation fibers (DCF), dispersion flatteningfibers, etc.

The Raman amplifier 100 is connected to a network 122 via acommunication link 123. The network 122 is also connected to otheramplifiers 124, 125 fiber as well as a remote device controller 121. Theremote device controller 121 monitors the operational status of theRaman amplifier 100 as well as the other amplifiers 124, 125. Thenetwork 122 may be a proprietary wireless or wired network, or anothernetwork that is publicly accessible, such as the Internet or a hybridnetwork, part proprietary and part publicly accessible. While the Ramanamplifier 100 may operate autonomously, it may receive additionalinformation about the overall system performance, such that the controlunit 119 can adapt the amplification performance of the Raman amplifier100 to help offset any adverse affects to the system's performance, asmight be necessitated by a change in conditions, described in theadditional information. As an example, this additional information maybe that a replacement fiber with different attenuation characteristicsis being used to interconnect two cascaded Raman amplifiers in a WDMsystem. In this case, the Raman amplifier 100 may set a new “target”amplification performance so as to normalize the channel characteristicsfor all of the WDM channels, despite the fact that the new fiber mayattenuate some of the channels by a lesser amount than others.

The pumping device 107 includes Fabry-Perot type semiconductor lasers109, 110, 111, 112, wavelength stabilizing fiber gratings 113, 114, 115,116, polarization couplers 117, 118, and a WDM coupler 108. The centralwavelengths of the semiconductor lasers 109 and 110 and wavelengths ofthe fiber gratings 113 and 114 are the same wavelength λ₁, and thecentral wavelengths of the semiconductor lasers 111 and 112 andreflection wavelengths of the fiber gratings 115 fiber and 116 are thesame wavelength λ₂. The central wavelengths of the semiconductor lasers109, 110, and 111 and 112 are respectively stabilized to λ₁ and λ₂ viathe wavelength stabilizing fiber gratings 113, 114 and 115, 116.

Multimode light generated by the semiconductor lasers 109, 110 and 111,112 is combined by polarization combiners 117, 118 for each centralwavelength λ₁ and λ₂, respectively. The light output from thepolarization combiners 117, 118 is combined by the WDM coupler 108.Polarization maintaining fibers 126 are used in the connections betweenthe semiconductor lasers 109, 110, 111, 112 and the polarizationcombiners 117, 118 to maintain two different polarization planes. Thisensures that an input signal to the Raman amplifier 100 will beadequately amplified regardless of its orientation in the signal fiber101 or amplification fiber 103.

The pumping device 107 in this example includes two pumps that providelight having two different wavelengths λ₁ and λ₂ to the amplifier fiber103 (i.e., a first pump that provides light having a central wavelengthof λ₁, and a second pump that provides light having a central wavelengthof λ₂). Further, as noted in U.S. Pat. No. 6,292,288, a wavelengthinterval between the wavelengths λ₁ and λ₂ is selected to be in a rangeof 6 nm to 35 nm in order to provide a flat gain profile over a rangeincluding both λ₁ and λ₂.

The light output from the pumping device 107 is coupled to the amplifierfiber 103 via the WDM coupler 104. An optical signal (e.g., a WDMsignal) is incident on the amplifier fiber 103 via the input fiber 101.The optical signal is then boosted in signal level after the gain mediumhas being excited by the light pumped into the amplifier fiber 103, thenet result being that the optical signal is Raman-amplified. Inaddition, the Raman-amplified optical signal is passed through the WDMcoupler 104 and is transmitted toward the control unit 119, where a partof the amplified optical signal is branched to form a monitor signal (orsampled output signal), while the majority of the signal is output onthe output fiber 106.

The control unit 119 includes a processor to assert control over theamplification performance of the Raman amplifier. The control can bebased on either the monitored signal or an external source, such as, forexample, a control signal received from the remote device controller121. The control unit 119 generates a control signal on a bus 120, thatincludes a sufficient number of control lines, so as to allow forcontrol of the drive currents and for the individual semiconductorlasers 109, 110, 111, 112 to achieve a small gain deviation relative toa target gain profile (e.g., a flat amplification profile).

As compared with EDFAs, for example, Raman amplifiers are more complexdevices since they contain more laser diode modules, operate over widerbandwidths that are determined by system parameters, and requirecontrollers that are able to establish predetermined amounts of gainacross the amplification bandwidth, consistent with networkrequirements. As recognized by the present inventors, part of thecomplexity is manifested in a controller that is able to adjust pumpoutput levels when environmental or network requirements change. Forexample, the central wavelengths of the pump modules will change as afunction of temperature. This change in central wavelength will resultin change in gain shape, which must be detected by the controller andcompensated. However, changing pump power level also effects the Ramanamplifier's gain characteristic, and thus optimum control is not alwayspossible due to temperature induced wavelength shifts in pump light.

Likewise, changes in system requirements may create a situation wherethe amplification bandwidth of the amplifier must be changed (i.e.,widen, or shift to another band). While some changes are possible byswitching-in or switching-out pumps to accommodate bandwidth changes,this leads to more expensive amplifiers, because more pumps are neededin the amplifier, albeit not used until needed. Likewise, some of thepumps may degrade overtime or fail. On-board spares may be used tomitigate reliability concerns, however this solution is expensive toimplement if all pumps are provided with an on-board spare.

The addition of channels within the amplification bandwidth is anotherpossible scenario. In such a scenario, it may be necessary to increaseamplifier power to avoid pump depletion. Since Raman scattering is anon-linear process, the amplifier gain cannot be increased by simplyincreasing the laser pump power, since doing so will change the powerpartitioning in the output wavelengths, which will likely result in anon-flat gain spectrum. Furthermore, the non-linear gain characteristicsmay give rise to undesirable four-wave mixing products, which may resultin in-band (signal band) noise spurs that compromise signal to noise (orspurious) ratio requirements. As recognized by the present inventors,the four-wave mixing products are the result of operating at too high ofan input pump light level at a particular pump central wavelength. Whenthis occurs, the amplification characteristic of the gain medium at thatcentral wavelength is substantially non-linear, and undesirable spuriousresponses are created. Given the number pumps that operate at differentwavelengths in a Raman amplifier, the risk of four-wave mixing can beintractable if not detected and controlled by reducing the pump levels.However, if reducing the pump levels results in insufficientamplification, the problem can be circumvented by reconfiguring the pumpwavelengths in such a way that they provide sufficient gain withsufficient flatness, and their four-wave mixing products are eithersuppressed or lie out-of-band.

SUMMARY OF THE INVENTION

The inventors of the present invention have recognized that conventionaloptical communication systems are limited as to their flexibility andadaptability. Accordingly, one object of the present invention is toprovide a tunable multimode wavelength division multiplex Raman pump andamplifier, and a system, method and computer program product forcontrolling the same, which address the above-identified and otherlimitations of conventional systems.

In order to achieve flat gain (or an arbitrary gain shape, such as tocompensate for a non-flat fiber attenuation characteristic) for a Ramanamplifier, as discussed above, the wavelengths and power levels of eachRaman pump are carefully chosen. The present inventors have recognizedthat, unfortunately, the term “flat gain” can mean different thingsdepending on the particular characteristics of a given communicationssystem. Furthermore, the present inventors have recognized that therequirements for a particular system may change over time. For example,new channels may be added or dropped at different wavelengths, bit ratesmay be increased, or in-line components that are not easily replaced mayfail. A one-size-fits-all approach to Raman pumping does not exist. Theinventors of the present invention have recognized that it would beadvantageous to have a Raman pump module that can be reconfigured bytuning the central wavelength and setting the optical output of eachRaman pump individually, allowing the controller to alter the gainprofile as needed. Accordingly, another object of the present inventionis to provide a tunable multimode Raman pump and a tunable Ramanamplifier that uses tunable stabilizer fiber Bragg gratings (FBG) toshift the reflection bandwidth. By providing a tunable Raman pump and atunable Raman amplifier, more flexible optical communication systems canbe built that can be controlled to adjust to either changes in therequirements of the network, or to sub-optimal performance caused by avariety factors.

To achieve these and other objects, the present invention provides atunable multimode WDM Raman pump, amplifier, control system, method andsoftware that uses a plurality of multimode pumps, whose optical outputsand central wavelengths are controlled by a control unit. Controllingboth the wavelength and optical output of the pumps to predeterminedlevels and/or wavelengths enables a flexible approach toward Ramanamplifying a WDM optical signal that propagates through the opticalfiber that serves as the Raman gain medium. The control unit ensuresthat the Raman amplification profile (e.g., a predetermined amplifiergain profile across the amplification bandwidth, and/or theamplification wavelength span) is set and maintained to be consistentwith system requirements.

The control unit monitors the amplified WDM signal and, subsequently,determines if the monitored amplified WDM signal is within apredetermined threshold of the target amplification profile. If theRaman-amplified signal is not within the predetermined threshold, thecontrol unit actively controls the pumps (by adjusting at least one ofthe optical output and central wavelength) to bring the monitoredamplified WDM signal within the predetermined threshold of the targetamplification profile. The control of the individual pumps may includeadjustments made to the output power of the pump and/or the outputwavelength of pumping light provided by the pump.

The control unit is also configured to respond to control signals froman external source (e.g., an central controller or other source) thatdirects the Raman amplifier to create a new target amplificationprofile. This new target amplification profile may be based on, forexample, a change in system operating conditions or system requirements.

The control unit is also configured to monitor for presence of four-wavemixing by observing in-band noise products that are more narrowband thanbackground noise. When detected, the control unit adjusts the centralwavelengths of the contributing pumps so as to “steer” the narrowbandnoise out of the signal band.

In one embodiment of the present invention a reconfigurable Raman pumpmodule includes a multiplexed array of wavelength tunable laser diodes.Tuning is achieved by uniformly shifting the reflection bandwidth of thepump's stabilizer FBG. This approach allows for a single module designthat is applicable for many different systems, thereby reducing themanufacturing and inventory problems associated with custom-builtequipment. It also reduces the costs associated with future systemupgrades.

One embodiment of the reconfigurable Raman pump includes a semiconductorlaser diode with an electronic input optically coupled to a waveguideoutput. A grating structure is inscribed in the waveguide output toprovide a small amount of feedback to the diode. The grating is coupledto a tuning mechanism that changes either the grating period, theeffective index of the waveguide in the grating region, or both. Thereflected feedback from the grating causes the lasing linewidth (orbandwidth of optical output) of the laser diode to roughly correspond tothe grating bandwidth. When the grating is tuned, its reflectionbandwidth uniformly shifts, resulting in a corresponding uniform shiftin the multimode output of the laser diode, thereby achieving tunabilityof the Raman pump.

The tuning of the grating in the context of the present invention may bethrough, for example, thermal effects or strain effects (compressive ortensile). Thermal tuning works by inducing changes in the effectiveindex of the waveguide, known as the thermo-optic effect. Changes in thegrating period due to the thermal expansion of the waveguide are asecond order effect, at least for silica-based materials. Strain tuningworks by both changing the effective index via the stress-optic effectand causing small changes in the grating period.

Another feature of the present invention is that each Raman amplifier inan optical communication system need not operate alone, but rather mayoperate in an internetworked fashion with other amplifiers in thesystem. Since Raman amplification is a distributed amplification, thepresent invention exploits this distributed effect by shiftingamplification duties between adjacent, cascaded Raman amplifiers so asto compensate for unforeseen changes in component operations or systemrequirements.

Consistent with the title of this section, the above summary is notintended to be an exhaustive discussion of all the features orembodiments of the present invention. A more complete, although notnecessarily exhaustive, description of the features and embodiments ofthe invention is found in the section entitled “DESCRIPTION OF THEPREFERRED EMBODIMENTS,” and more generally throughout the entiredocument.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a block diagram illustrating a conventional Raman amplifier;

FIG. 2 is a block diagram illustrating a Raman amplifier according toone embodiment of the present invention;

FIG. 3 is a high-level schematic diagram of a tunable laser diodeaccording to one embodiment of the present invention;

FIG. 4 is a schematic diagram of a heat-tuned grating package accordingto one embodiment of the present invention;

FIG. 5 is a schematic diagram of a strain-tuned grating packageaccording to one embodiment of the present invention;

FIG. 6 is an illustration of an apparatus for demonstrating astrain-tuned grating package according to one embodiment of the presentinvention;

FIG. 7 is a graph showing the transmission spectrum of a heat-tunedstabilizer grating at various tuning states according to one embodimentof the present invention;

FIG. 8 is a graph showing output power for a laser diode with astabilizer grating thermally tuned to various wavelengths according toone embodiment of the present invention;

FIG. 9 is a graph showing the transmission spectrum of a strain-tunedstabilizer grating at various tuning states according to one embodimentof the present invention;

FIG. 10 is a graph showing output power for a laser diode without astabilizer grating and with a stabilizer grating strain-tuned to variouswavelengths according to one embodiment of the present invention;

FIG. 11 is a schematic diagram of an amplifier module including aplurality of multiplexed tunable laser diodes according to oneembodiment of the present invention;

FIG. 12 is graph showing the simulated Raman gain for a 3 diode pumpmodule amplifying 40 channels according to one embodiment of the presentinvention;

FIG. 13 is a graph showing the simulated Raman gain for a 3 diode pumpmodule amplifying 80 channels with and without the wavelength tunabilityfeature according to one embodiment of the present invention;

FIG. 14 is a schematic diagram of a Vernier effect distributed feedback(DFB) laser including a semiconductor optical amplifier (SOA) gainmedium and two offset sampled gratings, where one of the sampledgratings is tunable according to one embodiment of the presentinvention;

FIG. 15 is a graph showing the reflection spectra of two offset sampledgratings according to one embodiment of the present invention;

FIG. 16 is a block diagram illustrating the details of the control unitfor a Raman amplifier according to one embodiment of the presentinvention;

FIG. 17 is a schematic illustrating components included in the controlunit according to the present invention;

FIG. 18 is a schematic illustrating other components included in thecontrol unit according to the present invention;

FIG. 19 is a schematic which illustrates controlling an output powerfrom a Raman amplifier by monitoring optical signals input to and outputfrom the Raman amplifier;

FIG. 20 is a schematic illustrating a computer system included in thecontrol unit according to the present invention;

FIG. 21 is a flowchart illustrating an exemplary high level controlprocess performed by a control unit according to one embodiment of thepresent invention;

FIG. 22 is a flowchart illustrating a control operation of the Ramanamplifier according to the present invention;

FIG. 23 is a graph illustrating a wavelength-dependency characteristicof fiber loss in an optical fiber;

FIG. 24 is a fiber loss data table used by the control unit according tothe present invention;

FIG. 25 is a graph illustrating a superposition principle for predictinga Raman amplification profile according to the present invention;

FIGS. 26A and 26B are graphs illustrating a design of a pumping devicebased on the superposition principle according to the present invention;

FIG. 27 is a graph illustrating a predicted Raman amplification profilebased on the superposition principle and an actual Raman amplificationprofile;

FIG. 28 is a schematic of another pumping device according to thepresent invention;

FIG. 29 is a schematic for explaining another Raman amplificationexample according to the present invention;

FIG. 30 is a graph illustrating amplification profiles of the pumpingdevice in FIG. 29;

FIG. 31 is a graph illustrating an enlarged view of a totalamplification profile of the pumping device in FIG. 29;

FIG. 32 is a graph illustrating amplification profiles for a variationof the pumping device in FIG. 29;

FIG. 33 is a graph illustrating an enlarged view of a totalamplification profile of the pumping device shown in FIG. 29;

FIG. 34 is a schematic for illustrating yet another Raman amplificationexample according to the present invention;

FIG. 35 is a graph illustrating amplification profiles of the pumpingdevice in FIG. 34;

FIG. 36 is an enlarged view of the total amplification profile shown inFIG. 35;

FIG. 37 is a schematic for explaining still another Raman amplificationexample according to the present invention;

FIG. 38 is a graph illustrating amplification profiles of the pumpingdevice in FIG. 28;

FIG. 39 is a graph illustrating an enlarged view of a totalamplification profile shown in FIG. 38;

FIG. 40 is a schematic for explaining another Raman amplificationexample according to the present invention;

FIG. 41 is a graph illustrating amplification profiles for a pumpingdevice including a bank of thirteen pumps;

FIG. 42 is a graph illustrating an enlarged view of a totalamplification profile shown in FIG. 41;

FIG. 43 is a graph illustrating amplification profiles for a variationof the pumping device in FIG. 41;

FIG. 44 is a graph illustrating an enlarged view of a totalamplification profile illustrated in FIG. 43;

FIG. 45 is a flowchart illustrating yet another control operationaccording to the present invention; and

FIG. 46 is a schematic of cascaded Raman amplifiers and an associatedcontrol unit according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 2 is a block diagram of a Ramanamplifier 200 according to one embodiment of the present invention. TheRaman amplifier 200 includes an amplification fiber (optical fiber) 103,a WDM coupler 104, a pumping device 201, a control unit 206, andoptional polarization independent isolators 102, 105. The Ramanamplifier 200 is connected to an input fiber 101 and an output fiber106, which may be optical transmission fibers such as single modefibers, dispersion compensation fibers (DCF), dispersion flatteningfibers, etc. The input fiber 101, amplification fiber 103 and outputfiber 106 may be the same fiber.

The Raman amplifier 200 is connected to a network 122 via acommunication link 123. The network 122 is also connected to otheramplifiers 207, 208 as well as a remote device controller 121. Theremote device controller 121 monitors the operational status of theRaman amplifier 200 as well as the other amplifiers 207, 208. Thenetwork 122 may be a proprietary wireless or wired network, or anothernetwork that is publicly accessible, such as the Internet or a hybridnetwork, part proprietary and part publicly accessible. While the Ramanamplifier 200 may operate autonomously, it may also be provided withadditional information about the overall system performance, such thatthe control unit 206 can adapt the amplification performance of theRaman amplifier 200 to help offset any adverse affects to the system'sperformance as indicated by a change in conditions, reflected in theadditional information. As an example, the additional information may bethat a replacement fiber with different attenuation characteristics isbeing used to interconnect two cascaded Raman amplifiers in a WDMsystem. In this case, the Raman amplifier 200 may employ a new “target”amplification performance so as to normalize the channel characteristicsfor all of the WDM channels, despite the fact that the new fiber mayattenuate some of the channels by a lesser amount than others.

The pumping device 201 includes Fabry-Perot type semiconductor lasers202, 203, 204, 205, tunable wavelength stabilizing fiber gratings 209,210, 211, 212, polarization couplers 117, 118, and a WDM coupler 108.The tunable fiber gratings 209, 210 are tuned to the same centralwavelength λ₁, and the tunable fiber gratings 211, 212 are tuned to thesame central wavelength λ₂.

The light output from the polarization combiners 117, 118 is combined bythe WDM coupler 108. Polarization maintaining fibers 126 are used in theconnections between the semiconductor laser devices 202, 203, 204, 205,the tunable fiber gratings 209, 210, 211, 212, and the polarizationcombiners 117, 118 to maintain two different polarization planes, thusensuring an input signal to the Raman amplifier 200 will be adequatelyamplified regardless of its orientation in the signal fiber 101 oramplification fiber 103.

The pumping device 201 in this example includes two pumps that providelight having wavelengths λ₁ and λ₂ to the amplifier fiber 103. However,since the pumping device 201 includes tunable fiber gratings 209, 210,211, 212, the two wavelengths λ₁ and λ₂ are adjustable, and need not bedifferent. Moreover, the semiconductor lasers need not be preconfiguredto generate light at any particular wavelength. In one embodiment of thepresent invention, the pumping devices 201 are configured in centralwavelength and optical output by the customer for a particularapplication. The inventors of the present invention have recognized thatby providing “field configurable” Raman amplifiers, customers have moreflexibility to adjust the operational performance of their system.Moreover, customers need not maintain an inventory of Raman amplifiersfor each configuration included in their system since the tunable Ramanpumps and amplifiers of the present invention are configurable by thecustomer. Thus, one “generic” Raman amplifier with tunable multimodeRaman pumps may be stocked by a system operator, and then programmed inthe field (locally at the control unit 206, or remotely via the remotedevice controller 121) to operate with a predetermined gain shape andamplification bandwidth.

The light output from the pumping device 201 is coupled to the amplifierfiber 103 via the WDM coupler 104. An optical signal (e.g., a WDMsignal) is incident on the amplifier fiber 103 via the input fiber 101.The optical signal is then combined with the light pumped into theamplifier fiber 103 so the incident optical signal is Raman-amplified.In addition, the Raman-amplified optical signal is passed through theWDM coupler 104 and is transmitted toward the control unit 206, where apart of the amplified optical signal is branched to form a monitorsignal (or sampled output signal), while the majority of the signal isoutput on the output fiber 106. The monitor signal is analyzed atseparate wavelengths by the control unit 206 to determine if the outputlevel of the optical signal complies with the intended level. If notcorrective action is taken to make an adjustment to the amplificationprofile. The controller may include other detection techniques as well,such as narrowband noise detection, so as to assess whether four-wavemixing is a problem, and if so to identify the offending pump laser.

The control unit 206 uses a processor to assert control over the outputpower and/or tune the central wavelength of light provided by each ofthe semiconductor lasers 202, 203, 204, 205 fiber by controlling thetunable fiber gratings 209, 210, 211, 212, thereby controlling theoverall amplification performance of the Raman amplifier 200. Thecontrol can be based on either the monitor signal or an external source,such as, for example, a control signal received from the remote devicecontroller 121. The control unit 206 generates a control signal on a bus120, that includes a sufficient number of control lines, so as to allowfor control of the drive currents to achieve a small gain deviationrelative to a target gain profile (e.g., a flat amplification profile),and/or control individual tunable fiber gratings 209, 210, 211, 212, toadjust the central wavelengths of the individual semiconductor lasers202, 203, 204, 205.

While there are a number of different embodiments of the control unit206, a common feature of each embodiment is that a processor is employedto assert control over the amplification performance and/or the centralwavelength of light provided by the Raman amplifier 200. While someembodiments include a signal monitoring feature in the control unit 206,it should be understood that the control unit 206 can receive amonitored output signal or other control information from an externalsource. For example, as shown in FIG. 2, a remote device controller 121may provide control information via the network 122. Likewise, thecontrol unit 206 may include a laser driver circuit, or simply aninterface to a driver circuit that is external to the control unit 206.In each case, however, the control unit 206 is equipped with a processorthat is able to execute a series of instructions (e.g., by way of a PAL,or ASIC) to interpret whether the output of the Raman amplifier 200 iswithin a predetermined tolerance of a target amplification performanceand/or a central wavelength of light output, and take corrective actionas appropriate. Furthermore, the control unit 206 is capable ofimplementing a change in the operational characteristics of the Ramanamplifier 200 based on control information provided to the control unit206.

U.S. patent application Ser. No. 09/775,632 describes how to control theoutput power of the Raman amplifier 200. It should be understood thatthe control unit 206 may include a software (or firmware/hardwareequivalent) output power control mechanism as described in Ser. No.09/775,632. However, the remainder of the description herein willprimarily focus on the control unit's 206 control over the centralwavelength of the output pumping light of the Raman amplifier 200through tuning of the tunable fiber gratings 209, 210, 211, 212.

The control unit 206 monitors the post-amplification signal andgenerates a control signal on a bus 120 that includes a sufficientnumber of control lines to control the semiconductor laser devices 202,203, 204, 205 (e.g., including control lines to control the drivecurrents of the semiconductor laser devices 202, 203, 204, 205 andcontrol lines to control the tunable fiber gratings 209, 210, 211, 212).For example, when the control unit 206 is configured to control both theoutput power and the wavelength of a Raman amplifier 200 including foursemiconductor laser devices 202, 203, 204, 205, at least eight controllines are included on the bus 120 (i.e., one drive current control lineand one fiber grating tuning control line for each of the foursemiconductor laser devices 202, 203, 204, 205). Alternatives arepossible as well where the control unit 206 outputs a digital signal onbus, which is interpreted by another processor (or firmware/hardwareequivalent) located near the drive circuits and gratings and convertedinto discrete control signals.

The control unit 206 performs the function of controlling the gainprofile of the multimode Raman amplifier 200, and then monitoring thesignal output from the Raman amplifier 200 to ensure the amplifier isactually operating within a predetermined tolerance of the target gainprofile. The control unit 206 will initially identify and set the targetcentral wavelengths of the semiconductor laser devices 202, 203, 204,205, but it is also configured to make adjustments to the target gainprofile of the amplifier 200 if it is provided with additionalinformation about the system-level performance that has not yet beentaken into account when establishing the target gain profile. Forexample, if a failure in a pump laser of an adjacent downstreamamplifier is reported to the control unit 206, the control unit 206 maybe able to compensate for this failure, which would manifest itself by aless than ideal amount of amplification in a portion of theamplification by increasing the amplification profile for that portionof the amplification band in the Raman amplifier 200.

A first functional feature of the control unit 206 is that it isconfigured to obtain a target Raman amplification performance and thenmonitor an actual output (Raman amplified) WDM signal to determine ifthe monitored amplification performance is within a predeterminedtolerance of the target performance. The control unit 206 performs thismonitoring operation by identifying deviations between the actualperformance and the target performance. If the control unit 206determines that the monitored amplification performance is within theallowable tolerance of the target amplification performance throughoutthe amplification band, the control unit 206 does not alter theconditions for the pumping device and continues monitoring the actualamplification performance. However, if the control unit 206 determinesthat the actual amplification is not within the allowable tolerance ofthe target amplification performance, the control unit may increase ordecrease pumping power and/or tune the central wavelengths of one ormore of the pumps so as to maintain the target amplificationperformance.

Another feature of the control unit 206 is that it is configured tochange the target performance as well as the conditions for a pumpingdevice to produce a new target performance for Raman amplification. Thisfunction may be performed as a configuration function when the Ramanamplifier 200 is initially installed, or may be performed in response toa change in system requirements. For example, a particular communicationband may need to be expanded to accommodate additional channels on thenetwork. To accommodate this change in system requirements, the controlunit 206 may be configured to set another set of conditions for thepumping device, such as turning on or off or changing the output poweror central wavelength of one or more of the semiconductor laser devices202, 203, 204, 205 included in the Raman amplifier 200 so as to achievea different target performance for increasing a communication bandwidth.

A third feature of the present invention is that the control unit 206 isnetworked with other amplifiers (e.g., amplifiers 207, 208) in acascaded arrangement, such that by coordinating amplification profilesbetween the cascaded amplifiers, the overall system performance remainsoptimum for WDM signals, despite the fact that the communicationconditions may have changed in some way from an original system design.For example, by coordinating the amplification profiles between adjacentamplifiers, the failure of selected pumps may be compensated for byadjusting the amplification profile of one or more adjacent amplifiers.For example, the control unit 206 may adjust the amplificationperformances of pre-cascaded and/or post-cascaded Raman amplifiers tooffset a problem that has arisen with a Raman amplifier connectedtherebetween. Other conditions may be compensated for as well, such asthe use of a different fiber, with different attenuation characteristicsthan the original fiber, the insertion of another network component(e.g., switch or amplifier) between two Raman amplifiers, or drifts in acentral wavelength of a Raman amplifier due to variations intemperature.

FIG. 3 is a schematic diagram of tunable diode device according to oneembodiment of the present invention. The solid state diode laser device1 is electrically pumped and lases over a fairly wide bandwidth (e.g.,5-20 nm), depending on an injection current provided to the laser device1. The light generated by the laser device 1 is optically coupled into awaveguide 2 which, for example, would be a length of a single modefiber. A tunable Bragg grating 3, such as the tunable fiber gratings209, 210, 211, 212 shown in FIG. 2, is written into the waveguide 2, andprovides optical feedback to the laser device 1. The grating 3,described in detail below, is located sufficiently far from the laserdevice 1 so that the feedback it provides is completely out of phasewith the light field in the cavity of the diode laser device 1. Thefeedback destroys the coherence of the laser cavity field; which in turncauses the laser to rearrange its spectral properties to maximize thefeedback and restore coherence. In doing so, the output of the laserdevice 1 spectrally collapses to lie within the bandwidth of the tunablegrating 3, which may be, for example, between 0.5 nm and 1 nm. This actsto both stabilize the center wavelength of the laser device 1 output,regardless of the bias current on the laser device 1, and increase theslope efficiency of the laser device 1, since all of the optical poweris confined to within a small bandwidth. When the grating is tuned, thediode laser device 1 output shifts in wavelength.

FIGS. 4 and 5 fiber are representative schematic diagrams of packagesthat employ two tuning mechanisms, thermal tuning and strain tuning, fortuning a tunable fiber grating 209, 210, 211, 212 according to differentembodiments of the present invention. The thermally tuned package shownin FIG. 4 includes a silica microcapillary tube 6 coated with a thinmetal film. This capillary is housed in a larger glass tube 4, with twoferrules 5, one on either side, to support the capillary 6 and theelectrical leads 7. When a voltage is applied across the thin resistivefilm, it dissipates the electrical power in the form of heat. Thecapillary tube 6 is sufficiently small such that it is power efficient,heats up quickly, and there are no radial thermal gradients inside of itwhen it is heated. Based on this configuration, it can be assured that afiber Bragg grating 8 placed inside this package will be uniformlyheated. The thermal tuning coefficient for silica is only 0.011 nm/C,which provides for about 2-3 nm of tunability. At higher temperatures,the grating will degrade. Strain tuning, and in particular, compressivestrain tuning, can achieve up to 20 times that range in the laboratory.Thermal tuning may nevertheless complement other tuning mechanism inwhich an external mechanical force is applied to the grating.

FIG. 5 illustrates an exemplary compact and reliable configuration forstrain tuning a fiber Bragg grating according to one embodiment of thepresent invention. In this embodiment, a tunability range of at least10-15 nm is achievable. As shown in FIG. 5, the apparatus includes alever package 9, made of a rigid material, that houses a piezoelectricactuator 10, the dimensions of which are matched to the package. Avoltage is applied to the leads 11 of the piezoelectric actuator 10causing it to expand horizontally. This expansion causes the two arms ofthe lever package 9 to push outward. A fiber Bragg grating 13 is mountedon the lever package 9 between the two arms, and is bonded on either endto, for example, two ferrules 12 that ensure proper alignment. The fiberBragg grating 13 is tuned as the outward motion of the two arms of thelever package 9 stretches it. Stretching the fiber corresponds totensile strain tuning, which has a much lower tuning range thancompressive strain tuning because it mechanically weakens the fiber.However, in another embodiment of the present invention, the packageshown in FIG. 5 is used for compressive strain tuning by bonding thefiber when the arms of the lever package 9 are extended. A compressivestrain increases when the piezoelectric is turned off (i.e., the voltageis removed from the leads 11). The gratings used for diode stabilizationare preferably very short (e.g., sub-millimeter) and therefore wellsuited for this sort of tuning. By compressing only a very short lengthof fiber, the likelihood of the fiber buckling is minimized.

FIG. 6 illustrates an apparatus though which one embodiment of thepresent invention has been demonstrated. As shown in FIG. 6, two plates13 are mounted on two separate stages 14, 15. The stage on the right 15can be adjusted along its x, y, and z axes with a set of threemicrometers 16. The stages 14, 15 may be electromechanical stages thatare controlled by, for example, stepper motors, MEMS or micromachinesthat are used to convert control signals into translation motion in theX, Y and Z axes. In one exemplary configuration, each mounting plate 13has a 180 μm radius semicircle trench running along its surface. Alength of silica capillary tubing 17 having, for example, a 250 μm innerdiameter and a 360 μm outer diameter is bonded into each trench using,for example, a heat-curable epoxy. The capillary tube mounted on theright hand stage 15 is aligned with respect to the tube mounted on theleft hand stage 14 along the x and z axes using a microscope. The y axismicrometers 16 is adjusted so that the gap between the two capillarytubes is 1.25 mm. A length of fiber 18 that has a 1 mm fiber Bragggrating 19 written in it is strung through the two tubes 17 and adjustedso that the grating 19 lies within the 1.25 mm gap. The grating 19 isthen bonded to the capillary tubes 17 by filling each tube with, forexample, a heat curable epoxy. The ends of capillary tubes 17 arecleaved before they are bonded to the plates 13 so that the bond linearound the fiber 18 is symmetric in the y direction. When the epoxy isfinished curing, the Bragg wavelength of the grating 19 can be tuned byadjusting the y-axis micrometer 16, which tenses or compresses thegrating region in the gap depending on the travel direction.

FIG. 7 is a graph showing transmission data for a heat-tuned stabilizergrating according to one embodiment of the present invention. Thegrating strength (4.4%) and bandwidth (˜1.1 nm) are typical for knownstabilizer gratings, although slightly stronger, weaker, wider, ornarrower gratings may also be used, depending on the application. FIG. 7illustrates transmission data at six different temperatures ranging from20° C. (identified by figure element 701) to 239° C. (identified byfigure element 706).

FIG. 8 is a graph showing the output spectrum of a diode laser with thegrating used to generate the graph of FIG. 7 providing feedback at thesame six temperatures according to one embodiment of the presentinvention.

FIG. 9 is a graph showing transmission data taken for a strain-tunedstabilizer grating according to one embodiment of the present invention.The grating strength in this example is 3% and the bandwidth is ˜1.7 nm.FIG. 9 illustrates transmission data at six different points rangingfrom 0.34% compressed (identified by figure element 901) through nostrain (identified by figure element 903) to 0.66% tensed (identified byfigure element 905).

FIG. 10 is a graph showing the output spectrum of a diode laser with thegrating used to generate the graph of FIG. 9 providing feedbackaccording to one embodiment of the present invention. The curveidentified by figure element 1001 shows the output of the diode withouta grating, has peak power of only 23 mW, and is quite broad. With thegrating, however, the peak power increases to 90 mW for the same biascurrent. The output spectrum is also significantly narrowed with thegrating, which is desirable for many applications, including use as apump in a Raman amplifier When the grating is strain-tuned, the mainlasing peak simply shifts over uniformly, as expected. The curves forvarious strain tunings (both compression and tensile) are illustrated bythe curves identified by figure elements 1002, 1003, 1004, 1005, 1006,and 1007.

FIG. 11 is an exemplary rough schematic diagram illustrating multipleRaman pumps being multiplexed onto a single transmission fiber accordingto one embodiment of the present invention. In this example a set offour 2×2 multiplexers is used.

FIG. 12 shows a simulated optimized Raman gain spectrum for athree-diode pump module according to one embodiment of the presentinvention. As shown in FIG. 12, 40 channels, spaced by 100 GHz, areamplified to transparency on a 100 km length of TRUEWAVE-reduced slopefiber with a gain ripple of only 1.18 dB. The wavelengths and outputpowers used in this example to yield these results were 1425.9 nm @332.7 mW, 1451 nm @ 335 mW, and 1470 nm @ 25 mW.

FIG. 13 is a graph illustrating that if 40 more channels at longerwavelengths were added to the system described in the context of FIG.12, the pump configuration would no longer be adequate. This isillustrated by the curve identified by figure element 1301, where thelonger wavelengths are not being amplified. The curve identified byfigure element 1303 illustrates an attempt to reconfigure the pumpmodule without using wavelength tuning (e.g., by adjusting the pumppowers). In order to amplify the longer wavelengths, the 1470 nm pumppower clearly needs to be increased. However, even when the 1470 nm lineis increased to 400 mW, the longest wavelengths are still not amplifiedto transparency. Furthermore, the gain ripple approaches 15 dB, and thesignal channels in the 1570 nm range are amplified so much thatnon-linearity problems such as self-phase modulation may arise. However,when the pump module is reconfigured using high wavelength tunability,as illustrated by the curve identified by figure element 1302,transparency can be achieved with a more reasonable gain ripple of 2.9dB. In this example, the favorable result was primarily enabled by thefact that the long wavelength pump was tuned to be placed at 1480 nmrather than 1470 nm.

FIG. 14 illustrates an alternate heat tuning approach for tuning a fibergrating according to one embodiment of the present invention. Heattuning can achieve a wider tunability than 2-3 nm when used with adifferent pump light source. Rather than using a diode laser, the lightsource used in the example illustrated in FIG. 14 is a semiconductoroptical amplifier 20 (SOA) which acts as a gain medium. Two sampledBragg gratings (SBG 1 21, SBG 2 22) placed on either side of thesemiconductor optical amplifier 20 form the laser cavity. Sampled Bragggratings, unlike conventional gratings, have multiple reflection peaks,the spacing between which is determined by the sampling period.Representative reflection spectra for the sampled Bragg gratings 21, 22are illustrated in the graph of FIG. 15. Lasing in this device onlyoccurs when two of the reflection peaks are aligned with one another.However, by using a tunable sampled Bragg grating for the second sampledBragg grating 22, the tuning of the SBG 2 22 causes discrete jumps inthe output wavelength of the laser. For example, if the second sampledBragg grating 22 were tuned by 0.2 nm, the wavelength at which one ofits peaks is aligned with the first sampled grating 21 could shift by 1nm from the original wavelength. Accordingly, the tuning efficiency ofthis device is quintupled, making heat tuning a viable option forachieving wide tunability.

In some embodiments of the present invention, the tunability range ofthe fiber grating exceeds the bandwidth of light provided by the laserdevice. In one of these embodiments, when it is desirable to tune thefiber grating beyond the bandwidth of light provided, the laser deviceis switched out and substituted with another laser device that provideslight that will be reflected by the tuned fiber grating, therebyallowing the full tunable range of the tunable fiber grating to used. Inanother embodiment, multiple diode/grating pairs having differeenttunability ranges are switched in and out to achieve a wider range oftuning. In this embodiment, the control algorithmn is simplified andlosses of optical power are avoided.

FIG. 16 is a block diagram illustrating the details of the control unit206 for a Raman amplifier according to one embodiment of the presentinvention. As shown in FIG. 16, the control unit 206 includes a tapcoupler 504 that is connected to a fiber grating wavelength tuningcontrol unit 502 and a laser device power control unit 503 through anoptical fiber that carries a fraction of the WDM optical signal to boththe fiber grating wavelength tuning control unit 502 and the laserdevice power control unit 503. While the tap coupler 504 is shown to behoused within the control unit 206, it may also be an external componentthat connects to the control unit 206. Furthermore, the separatecomponents of the control unit 206 (i.e., the fiber grating wavelengthtuning control unit 502, the laser device power control unit 503 and thecontroller 501) may be discrete components that need not be housedwithin a common control unit enclosure. Furthermore, the variouscomponents shown in FIG. 16 may be designed as either separate units, oras units made up of some combination thereof.

The fiber grating wavelength tuning control unit 502 and the laserdevice power control unit 503 demultiplex the WDM monitor signal (i.e.,the portion of the amplified WDM signal provided by the tap coupler504), and then convert the demultiplexed signals into electricalsignals. Samples of the electrical signals are provided to thecontroller 501 for performing the amplification control processes (e.g.,to adjust an amplitude and/or wavelength of the individual semiconductorlasers 202, 203, 204, 205). Since one of the functions performed by thecontrol unit 206 is to monitor the actual amplification performance ofthe Raman amplifier 200, the fiber grating wavelength tuning controlunit 502 and the laser device power control unit 503 sample theelectrical signals and compare this series of samples against a targetamplification performance. The sampling process performed by the fibergrating wavelength tuning control unit 502, the laser device powercontrol unit 503, and the controller 501 does not necessarily have to beperformed on a WDM channel-by-WDM channel basis. Rather, these units mayperform the control processes with greater or a lesser spectralresolution than one sample set per WDM channel. The fiber gratingwavelength tuning control unit 502 and the laser device power controlunit 503 provide output control lines 120 for controlling thewavelengths and optical output levels of each pump laser 202, 203, 204,205, respectively. The fiber grating wavelength tuning control unit 502and the laser device power control unit 503 also exchange sample dataand control information with the controller 501. A controller 501 isconfigured to connect to a data communication network 122 such as theInternet for exchanging data and control information with, for example,a remote device controller 121 and other amplifiers 207, 208.

FIGS. 17 and 18 provide more detailed descriptions of subcomponents ofthe fiber grating wavelength tuning control unit 502. In FIG. 17, thefiber grating wavelength tuning control unit 502 includes a wavelengthdemultiplexer 18, an optical/electrical converting mechanisms 19 (e.g.,photo-diodes) and an fiber grating wavelength tuning control circuit 20connected in series. The wavelength demultiplexer 18 separates themonitored WDM optical signal into a plurality of optical sample-signals,each having a different central wavelength. The demultiplexed opticalsample may, for example, correspond to channels of the WDM signal, asdiscussed above. Once again, the function performed by the demultiplexer18 is to isolate separate spectral components of the WDM signal that isbeing amplified by the Raman amplifier 200. For basic control schemes,the de-multiplexer 18 may only provide two sample-signals, perhaps oneat shorter wavelengths within the amplification band and another atlonger wavelengths in the amplification band. A limitation, however,with having too few sample-signals is that the resolution of the sampledsignal is not sufficient to observe sub-bands where the gain profile ofthe Raman amplifier is not within a predetermined tolerance (e.g., 1 dB)of the target amplification performance. On the other hand, having toomany sample-signals unnecessarily increases the expense and complexityof the processing resources in the control unit 206. Thus, as apractical guideline, the number of sample-signals to be developed is setto correspond with either a number of WDM channels to be handled by theRaman amplifier 200, or a number of pump lasers 202, 203, 204, 205employed in the Raman amplifier 200. Thus, typical numbers ofsample-signals developed by the de-multiplexer for dense WDM signalswill range between about 10 to 100. However, smaller numbers, such as 2mentioned above, or up to, or exceeding, 1000 are possible as well.

The optical/electrical converting mechanisms 19 convert thedemultiplexed optical sample signals into electrical signals. Outputcurrents provided by the converting mechanisms 19 vary depending on therespective magnitudes of the demultiplexed sample-signals. Thecontroller 501 receives the electrical currents via a bus 505, where thecontroller 501 then samples the respective currents to create a digitalrendition of the sample-signals. Alternatively, the controller 501receives the digital rendition of the sample-signals from the fibergrating wavelength tuning control circuit 20 which digitizes thesample-signals. Likewise, the converting mechanisms 19 provide adigitized output.

The fiber grating wavelength tuning control circuit 20 is shown to be aseparate controller from controller 501, but the two can be incorporatedinto a single processor-based controller. As shown in FIG. 17, however,the controller 501 is configured to implement a digital signal processorbased-embedded controller, while the main analog processing is performedin the fiber grating wavelength tuning control circuit 20. For example,in one embodiment of the present invention, the controller 501 holds inmemory appropriate tuning values corresponding to a certain strainand/or heat tuning of the individual tunable fiber gratings 209, 210,211, 212 that achieves a particular target central wavelength to beprovided by the individual pump lasers 202, 203, 204, 205. Once thetuning values are identified, the controller 501 then informs the fibergrating wavelength tuning control circuit 20 (either via a digitalmessage or separate analog signals), so the fiber grating wavelengthtuning control circuit 20 may control the tunable fiber gratings 209,210, 211, 212 to produce the desired pump laser wavelength. However, inanother embodiment, the fiber grating wavelength tuning control circuit20 may operate digitally and may itself hold in memory the tuning valuesthat are associated with achieving the target output central wavelength.In this case, the fiber grating wavelength tuning control circuit 20dispatches control signals to the tunable fiber gratings 209, 210, 211,212, which contain their own wavelength tuning circuits that respond tothe control signals or are interconnected with separate wavelengthtuning circuits.

FIG. 18 illustrates another exemplary embodiment of the control unit206. Unlike the embodiment of FIG. 17, the embodiment of FIG. 18includes a power splitter 21 and bandpass filters 22. The power splitter21 splits the monitored WDM optical signal branched by the tap coupler504 into a plurality of sample-signals. For example, the power splitter21 may be configured to divide the branched WDM signal into acorresponding number of channels of the WDM signal. The bandpass filters22 have different central wavelengths and fixed-width passbands thatonly permit the portion of the respective sample-signals having opticalenergy within the specific passband to pass therethrough. Theoptical/electrical converters 19, controller 501 and fiber gratingwavelength tuning control circuit 20 are like that described above inreference to FIG. 17.

It should be noted that although the discussion has been primarilyfocused on sampling the amplified output from the Raman amplifier 200 toperform the control operation, the control unit 206 may also sample theinput signal to the Raman amplifier 200, as shown in FIG. 19. Bydirectly measuring the input optical signal and the output opticalsignal, the control unit 206 is able to establish a direct measurementof amplifier gain, and the profile of the amplification gain. As analternative to measuring the profile of the amplification gain, thecontroller 501 may receive a message from a downstream Raman amplifierwhich describes an output level of the WDM signal as it leaves Ramanamplifier. Since the fiber loss characteristics are generally known forthe fiber that interconnects two amplifiers, the controller 501 in thedownstream amplifier can calculate the apparent level of the WDM signalthat is input to that downstream Raman amplifier.

FIG. 20 illustrates an exemplary embodiment of a processor basedcontroller 501. The controller 501 includes a bus 902 or othercommunication mechanism for communicating information, and a processor903 coupled with the bus 902 for processing the information. Thecontroller 501 also includes a main memory 904, such as a random accessmemory (RAM) or other dynamic storage device (e.g., dynamic RAM (DRAM),static RAM (SRAM), and synchronous DRAM (SDRAM)), coupled to the bus 902for storing information and instructions to be executed by processor903. In addition, the main memory 904 may be used for storing temporaryvariables or other intermediate information during the execution ofinstructions by the processor 903. The controller 501 further includes aread only memory (ROM) 905 or other static storage device (e.g.,programmable ROM (PROM), erasable PROM (EPROM), and electricallyerasable PROM (EEPROM)) coupled to the bus 902 for storing staticinformation and instructions for the processor 903.

The controller 501 also includes a disk controller 906 coupled to thebus 902 to control one or more storage devices for storing informationand instructions, such as a magnetic hard disk 907, and a removablemedia drive 908 (e.g., floppy disk drive, read-only compact disc drive,read/write compact disc drive, compact disc jukebox, tape drive, andremovable magneto-optical drive). The storage devices may be added tothe controller 501 using an appropriate device interface (e.g., smallcomputer system interface (SCSI), integrated device electronics (IDE),enhanced-IDE (E-IDE), direct memory access (DMA), or ultra-DMA).

The controller 501 may also include special purpose logic devices (e.g.,application specific integrated circuits (ASICs)) or configurable logicdevices (e.g., simple programmable logic devices (SPLDs), complexprogrammable logic devices (CPLDs), and field programmable gate arrays(FPGAs)).

The controller 501 may also include a display controller 909 coupled tothe bus 902 to control a display 910, such as a cathode ray tube (CRT),for displaying information to a computer user. The computer systemincludes input devices, such as a keyboard 911 and a pointing device912, for interacting with a computer user and providing information tothe processor 903. The pointing device 912, for example, may be a mouse,a trackball, or a pointing stick for communicating direction informationand command selections to the processor 903 and for controlling cursormovement on the display 910. In addition, a printer may provide printedlistings of data stored and/or generated by the controller 501.

The controller 501 performs a portion or all of the processing steps ofthe invention in response to the processor 903 executing one or moresequences of one or more instructions contained in a memory, such as themain memory 904. Such instructions may be read into the main memory 904from another computer readable medium, such as a hard disk 907 or aremovable media drive 908, or downloaded from another processor, forexample, the remote device controller 121. One or more processors in amulti-processing arrangement may also be employed to execute thesequences of instructions contained in main memory 904. In alternativeembodiments, hard-wired circuitry may be used in place of or incombination with software instructions. Thus, embodiments are notlimited to any specific combination of hardware circuitry and software.

As stated above, the controller 501 includes at least one computerreadable medium or memory for holding instructions programmed accordingto the teachings of the invention and for containing data structures,tables, records, or other data described herein. Examples of computerreadable media are compact discs, hard disks, floppy disks, tape,magneto-optical disks, PROMs (EPROM, EEPROM, flash EPROM), DRAM, SRAM,SDRAM, or any other magnetic medium, compact discs (e.g., CD-ROM), orany other optical medium, punch cards, paper tape, or other physicalmedium with patterns of holes, a carrier wave (described below), or anyother medium from which a computer can read.

Stored on any one or on a combination of computer readable media, thepresent invention includes software for controlling the controller 501,for driving a device or devices for implementing the invention, and forenabling the controller 501 to interact with a human user (e.g., printproduction personnel). Such software may include, but is not limited to,device drivers, operating systems, development tools, and applicationssoftware. Such computer readable media further includes the computerprogram product of the present invention for performing all or a portion(if processing is distributed) of the processing performed inimplementing the invention.

The computer code devices of the present invention may be anyinterpretable or executable code mechanism, including but not limited toscripts, interpretable programs, dynamic link libraries (DLLs), Javaclasses, and complete executable programs. Moreover, parts of theprocessing of the present invention may be distributed for betterperformance, reliability, and/or cost.

The term “computer readable medium” as used herein refers to any mediumthat participates in providing instructions to the processor 903 forexecution. A computer readable medium may take many forms, including butnot limited to, non-volatile media, volatile media, and transmissionmedia. Non-volatile media includes, for example, optical, magneticdisks, and magneto-optical disks, such as the hard disk 907 or theremovable media drive 908. Volatile media includes dynamic memory, suchas the main memory 904. Transmission media includes coaxial cables,copper wire and fiber optics, including the wires that make up the bus902. Transmission media also may also take the form of acoustic or lightwaves, such as those generated during radio wave and infrared datacommunications.

Various forms of computer readable media may be involved in carrying outone or more sequences of one or more instructions to processor 903 forexecution. For example, the instructions may initially be carried on amagnetic disk of a remote computer. The remote computer can load theinstructions for implementing all or a portion of the present inventionremotely into a dynamic memory and send the instructions over atelephone line using a modem. A modem local to the controller 501 mayreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto the bus 902 can receive the data carried in the infrared signal andplace the data on the bus 902. The bus 902 carries the data to the mainmemory 904, from which the processor 903 retrieves and executes theinstructions. The instructions received by the main memory 904 mayoptionally be stored on storage device 907 or 908 either before or afterexecution by processor 903.

The controller 501 also includes a communication interface 913 coupledto the bus 902. The communication interface 913 provides a two-way datacommunication coupling to a network link 123 that is connected to, forexample, a local area network (LAN) 915, or to another communicationsnetwork 122 such as the Internet. The communication interface 913 alsoprovides a two-way coupling to the fiber grating wavelength tuningcontrol unit 502 and the laser device power control unit 503 via the bus505. The communication interface 913 may be a network interface card toattach to any packet switched LAN. As another example, the communicationinterface 913 may be an asymmetrical digital subscriber line (ADSL)card, an integrated services digital network (ISDN) card or a modem toprovide a data communication connection to a corresponding type ofcommunications line. Wireless links may also be implemented. In any suchimplementation, the communication interface 913 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

The network link 123 typically provides data communication through oneor more networks to other data devices. For example, the network link123 may provide a connection to a another computer through a localnetwork 915 (e.g., a LAN) or through equipment operated by a serviceprovider, which provides communication services through a communicationsnetwork 122. The local network 123 and the communications network 122use, for example, electrical, electromagnetic, or optical signals thatcarry digital data streams. The signals through the various networks andthe signals on the network link 123 and through the communicationinterface 913, which carry the digital data to and from the controller501, are exemplary forms of carrier waves transporting the information.The controller 501 can transmit and receive data, including programcode, through the network(s) 915 and 122, the network link 123 and thecommunication interface 913. Moreover, the network link 123 may providea connection through a LAN 915 to a mobile device 917 such as a personaldigital assistant (PDA) laptop computer, or cellular telephone.

FIG. 21 is a flowchart illustrating an exemplary high level controlprocess performed by the control unit 206 according to one embodiment ofthe present invention. As shown in FIG. 21, the process begins with stepS2101 where a gain profile requirement is received by the control unit206. As discussed above, the gain profile may be stored locally in thecontrol unit 206, or may be received, for example, from a remote devicecontroller 121 via the network connection 123 to the network 122. Thegain profile requirement may be an initial gain profile requirement, ormay reflect a change in a current gain profile requirement. The gainprofile requirement identifies a target performance for one or morepumping devices 201 under the control of the control unit 206.

Once the gain profile requirement has been received, the processproceeds to step S2102, where it is determined whether the current gainprofile matches the gain profile received in step S2101. If it isdetermined that the current gain profile matches the received gainprofile (i.e., “Yes” at step S2102), the process ends. If, on the otherhand, it is determined that the current gain profile does not match thereceived gain profile requirement (i.e., “No” at step S2102), theprocess proceeds to step S2103. At step S2103, the control unit 206generates control signals and transmits them via the bus 120 to one ormore pumping devices 201 to tune the wavelengths of one or more pumpsources 202, 203, 204, 205 to achieve the required gain profile. If thepumping device 201 includes more than one pump source 202, 203, 204,205, the control signals generated by the control unit 206 will bedirected to the appropriate pump source 202, 203, 204, 205 to tune thewavelengths of those individual pump sources 202, 203, 204, 205.

In one embodiment of the present invention, the tuning of the individualpump sources 202, 203, 204, 205 provided by the control unit 206 is acontinuous tuning capability. The tuning implemented in some cases is a“fine tuning” where, for example, a particular pump source 202, 203,204, 205 has drifted from its targeted central wavelength and iscorrected to its proper wavelength. In other cases, the control unit 206implements an entirely new gain profile that requires a significantchange in the central wavelength of one or more of the pump sources 202,203, 204, 205.

Once the wavelengths have been tuned, the process proceeds to step S2104where the control unit 206 similarly adjusts the output power of one ormore pump sources 202, 203, 204, 205 in order to achieve the requiredgain profile. As with the tuning of the wavelengths, the control signalswill be provided to the appropriate pump sources 202, 203, 204, 205 viathe bus 120. After the wavelengths and output power of the appropriatepump sources 202, 203, 204, 205 have been adjusted, the process proceedsto step S2105 where, as in step S2102, the current (i.e., adjusted) gainprofile is compared with the required gain profile received in stepS2101. If it is determined that the current gain profile matches therequired gain profile (i.e., “Yes” at step S2105), the process ends. If,on the other hand, it is determined that the current gain profile doesnot match the required gain profile (i.e., “No” at step S2105), theprocess returns to step S2103 where the wavelengths and output power ofappropriate pump sources 202, 203, 204, 205 are further tuned andadjusted.

FIG. 22 is a flowchart illustrating an operational process performed bythe control unit 206. This control process is followed for (1)establishing a predetermined target amplification performance (e.g., anamplification profile or output signal power profile over apredetermined amplification bandwidth), (2) monitoring whether an actualamplification performance is within a predetermined tolerance of thetarget amplification performance, and (3) taking corrective action whenthe actual amplification performance is not within the predeterminedtolerance. In particular, steps S2, S4 and S6 respectively identify thetarget output amplification performance, determine the amplifierparameters (e.g., pump laser central wavelengths) that are used toachieve the target output amplification performance and apply theparameters to achieve the target performance. Steps S8, S10, S12, andS14 are directed to ensuring that the actual amplification performancestays within a predetermined tolerance of the target amplificationprofile and/or at least adopts an acceptable profile shape (such as aflat or tilted profile slope).

As shown in FIG. 22, the process begins with step S2 where an input WDMoptical signal characteristic (e.g., a mean optical signal levelmeasured in sub-bands of the amplification band) and an (initial) targetamplification performance are provided to, and/or generated by, thecontrol unit 206 and stored in a memory, for example, the main memory904. The target amplification performance may be represented by a seriesof values indicative of a predetermined gain profile that is set by asystem operator to achieve a desired system performance. As would beunderstood by those of ordinary skill in the optical communication art,the gain profile includes both wavelength and amplitude information.Since gain relates to the level of an output signal relative to an inputsignal, the input signal level is identified either directly orindirectly, as will be discussed below. Moreover, if the system operatorintends to have the Raman amplifier operate with a predetermined gainprofile, the input WDM optical signal characteristic is first determinedin one of several ways, discussed below.

The WDM signal level may be measured directly at an input to the Ramanamplifier 200. In this case, the control unit 206 can determine whetherthe target gain is achieved by comparing the target gain to a ratio of ameasured output signal from the Raman amplifier 200 to the level of theoptical signal applied to the Raman amplifier 200. As an alternative toa direct measurement of the input signal level, the signal level may beobtained from an output signal level reported to the Raman amplifier 200from a downstream Raman amplifier (e.g., other amplifiers 207, 208),less an estimated, or measured, amount of attenuation due to fiber lossbetween the two Raman amplifiers. Still further, the input level may beinferred from a process employed by the control unit 206 in which asignal of known power is input to the Raman amplifier 200 and then anamount of driving current applied to respective pump lasers to produce apredetermined output level measured at the output of the subject Ramanamplifier 200 is identified and saved in memory. Subsequently, theamount of amplification may be estimated from a change in the amount ofdriving current relative to the values stored in memory. This latterprocess may be performed as an initial step during a built-in testprocedure or during a calibration operation, for example. As anotheralternative, a target output WDM optical signal characteristic may beprovided from an external source and stored in memory instead of thetarget amplification performance. In this case, the target amplificationperformance is calculated from an input WDM optical signalcharacteristic and a target output WDM optical signal characteristic andstored in a memory of the controller 501. The target amplificationperformance and input WDM optical signal characteristic may be provided,stored, and read, for example, from the main memory in the control unit206 during operational conditions.

After the target amplification performance, as well as associatedparameters are obtained in step S2, the process then proceeds to stepS4. In step S4, the amplifier parameters to achieve the targetamplification performance are determined by contemporaneouscalculation/simulation, or by referencing a look-up table that holdsparameters that were previously determined and stored for variousconditions. In the present discussion, the amplifier parameters will bedescribed as tunable fiber grating control values that are used to tunethe tunable fiber gratings 209, 210, 211, 212, by, for example,straining or heating the tunable fiber gratings 209, 210, 211, 212.

While the present discussion has focused on associating tunable fibergrating wavelength tuning control values with a target amplificationperformance, there are additional operational conditions that thecontrol unit 206 may consider, such as internal losses inherent in theoptical signal measurement operation, fiber losses, pump-to-pumpinteraction or aging of the pump lasers. To compensate for theseadditional conditions, such as fiber loss (attenuation), sets of tunablefiber grating wavelength tuning control values are pre-set and stored inmemory. The sets of tunable fiber grating wavelength tuning controlvalues correspond with various gain profiles that are available forselection so as to compensate for the fiber loss, etc.

By way of example as shown in the Nov. 28, 2000 publication “Photonics”by CIBC World Markets, FIG. 23 shows that it is known that an amount ofattenuation in an optical fiber at 1400 nm is much greater than at 1500nm. The conventional approach for dealing with this difference inattenuation is to simply use the part of the spectrum that has minimalattenuation. The present inventors take a different approach byemploying an amplification profile that compensates for the non-uniformattenuation characteristics in the transmission band. As seen in FIG.24, the main memory 904 holds a greater driving current (560 mA) for thepump laser having a peak Raman gain at 1400 nm, than the driving current(100 mA) for the pump having a peak Raman gain at 1500 nm, where thefiber loss is much less. Thus, by using a table such as that shown inFIG. 24, the driving currents can be determined so as to achieve thetarget amplification performance. Other parameters that affect thetarget profile, such as the tunable fiber grating wavelength tuningcontrol values corresponding to, for example, an amount of strain orheat to apply to the tunable fiber grating 209, 210, 211, 212, can alsobe stored in a data table in the memory of the controller 501.

There are simulation programs (e.g., OptSim by ARTIS Software)commercially available which can initially calculate the necessarytunable fiber grating wavelength tuning control values to achieve thedesired output profile. For example, the optical power around aparticular frequency in a WDM system may be expressed by the followingequation: $\begin{matrix}\begin{matrix}{\frac{P_{v}^{\pm}}{v} = {{{- \alpha_{v}}P_{v}^{\pm}} + {ɛ_{v}P_{v}^{\pm}} + {P_{v}^{\pm}{\sum\limits_{\mu > v}^{\quad}\quad {\frac{g_{\mu \quad v}}{A_{\mu}}\left( {P_{\mu}^{+} + P_{\mu}^{-}} \right)}}} +}} \\{{{2h\quad v{\sum\limits_{\mu > v}^{\quad}\quad {\frac{g_{\mu \quad v}}{A_{\mu}}{\left( {P_{\mu}^{+} + P_{\mu}^{-}} \right)\left\lbrack {1 + \frac{1}{{\exp \left\lbrack \frac{h\left( {\mu - v} \right)}{k\quad T} \right\rbrack} - 1}} \right\rbrack}}}} -}} \\{{{P_{v}^{\pm}{\sum\limits_{\mu < v}^{\quad}{\frac{v}{\mu}\frac{g_{{v\quad \mu}\quad}}{A_{v}}\left( {P_{\mu}^{+} + P_{\mu}^{-}} \right)}}} -}} \\{{4h\quad vP_{v}^{\pm}{\overset{\quad}{\sum\limits_{\mu < v}}\quad {\frac{g_{v\quad \mu}}{A_{v}}\left\lbrack {1 + \frac{1}{{\exp \left\lbrack \frac{h\left( {v - \mu} \right)}{k\quad T} \right\rbrack} - 1}} \right\rbrack}}}}\end{matrix} & (1)\end{matrix}$

where subscripts μ and ν denote optical frequencies, superscripts “+”and “−” denote forward- and backward-propagating waves, respectively,P_(ν) is optical power around ν, which is the product of power spectraldensity at ν and infinitesimal bandwidth dν.α_(ν) is attenuationcoefficient, ε_(ν) is Rayleigh backscattering coefficient, A_(ν) iseffective area of optical fiber at frequency ν, g_(μν) is a Raman gainparameter at frequency ν due to the pump at frequency μ, h is Planck'sconstant, k is Boltzmann constant, and T is temperature. This equationis expected to include almost all conceivable effects observable in realsystems, such as pump-to-pump and signal-to-signal Raman interactions,pump depletions due to Raman energy transfer, Rayleigh backscattering,fiber loss, spontaneous emission noise and blackbody radiation noise. Inone embodiment of the present invention, the main memory 904 of thecontroller 501 holds computer readable instructions that have the aboveequation (1) encoded therein. These instructions are executed by theprocessor 903 so as to simulate a target amplification performance basedon the aggregate amplification effects provided by the different pumplasers 202, 203, 204, 205.

FIGS. 25, 26A, 26B, and 27 will now be referred to when explaining anexemplary process of how to set a target amplification performanceincluding both amplitude and wavelength of step S4 in FIG. 22. FIGS. 25,26A, 26B and 27 illustrate a superposition principle that is used in oneembodiment according to the present invention to determine theappropriate conditions (e.g., center wavelength of pumps, pump outputpowers) needed to obtain a flat (or arbitrarily shaped) Ramanamplification performance.

FIG. 25 is a graph showing individual and composite Raman gain profilesversus wavelength for two pump lasers, operating at 1424.2 nm and 1451.8nm. The pump lasers may be referred to as YYXX lasers (YY being in arange of 13 through 15 and XX being in a range of 00 through 99). As anexample, for producing a Raman gain in the S-Band through L-band, theYYXX lasers may be referred to as 14XX pump lasers (e.g., 1400 nm to1499 nm). The Raman amplification gain profile due to the single pumplaser operating at a central wavelength of 1424.2 nm is shown as profile“a” in FIG. 25. Likewise, an amplification profile due to a single pumplaser operating at a central wavelength of 1451.8 nm is shown as profile“b”. The total Raman amplification profile due to the simultaneousoperation of both pumps is shown as profile “c” and is determined viathe superposition principle (i.e. the profiles are additive). That is,according to the superposition principle, the amplification profiles dueto each of the pumps may be added to achieve a total amplificationprofile that corresponds to the addition of the two individual profiles.

FIGS. 26A and 26B illustrate another example of the superpositionprinciple as applied to the present invention with regard to creatingtarget amplification performances. As shown in FIG. 26A, four pumplasers are tuned to the shorter wavelengths (i.e., a first group) andset at a first predetermined gain level (or optical output level), and afifth pump laser (i.e., a second group, having only one pump laser inthis example, but more could be included) is set to a higher gain level.The fifth pump laser is tuned to be separated in wavelength from theclosest of the pump lasers in the first group by a greater wavelengthinterval than between that of adjacent members in the first group.Moreover, the pump lasers in the first group are set to approximatelyequal gain levels and are tuned to be separated from one another byabout 20 nm (although a range of 6 nm to 35 nm is a reasonable separaterange to minimize appreciable inflection points in the gain profile). Inthis example, the fifth pump laser is tuned to operate at a centralwavelength of 1495.2 nm (29.2 nm above the closest pump laser in thefirst group, which operates at 1466.0 nm), and is set to impart aneffective gain that is almost 3 times higher than that of each of thefirst group of pumps.

FIG. 26B corresponds with FIG. 26A and illustrates how the superpositionprinciple applies to the amplification profiles produced by the pumplasers in the first group and in the second group. Amplificationprofiles “a”, “b”, “c” and “d” correspond with the pump lasers in thefirst group, which were respectively tuned operate at 1424.2 nm, 1437.9nm, 1451.8 nm and 1466.0 nm. Profile “g” corresponds with a compositeprofile for the first group and profile “e” corresponds with profileprovided by the pump laser in the second group, namely 1500 nm (althougha plurality of pumps, such as two or three, may be tuned to form thesecond group). Note that the respective amplification peaks occur at awavelength that is about 100 nm longer than where the source pumpoperates.

Target amplification gain profiles “c” (in FIG. 25) and “f” (in FIG.26B) are generated by applying the superposition principle. Theresulting shape of the predicted amplification gain profiles may be madesubstantially flat, as shown, or set to any arbitrary shape, byadjusting the outputs of the pump lasers, and tuning them to have aspecific spacing in the central wavelength at which the pump lasersoperate. For example, the amplification profile “f” shown in FIG. 26Bmay be made to tilt so as to have a negative slope throughout theamplification band by reducing the gain of the second group, namely thepump tuned to operate at the central wavelength of 1500 nm. Likewise,the profile could also be tilted to assume a negative slope byincreasing the output from group 1 (i.e., gain profile “g”). Conversely,target amplification profiles “c” (FIG. 25) and “f” (FIG. 26B) could bemade to have a positive slope by either reducing the output from thefirst group, and/or increasing the output from the second group. Gainprofiles “g” and “e” (FIG. 26B), which are provided by a group of pumps,may be referred to as “element gain profiles.” Since there are generallya greater number of pump lasers operating in the group(s) at the shorterwavelengths, it is possible to impart a greater number of higher ordergain shape features in the element gain profile for the shorterwavelengths than for the longer wavelengths.

After step S4 is completed, the process proceeds to step S6, where thecontrol unit 206 may then assert control over the optical output of thepump lasers by applying the amplification parameters (e.g., amplitudeand wavelength) previously determined to the pump lasers. As an example,when the simulated amplification performance matches the targetamplification performance, within a predetermined tolerance, respectivedrive currents and tunable fiber grating wavelength tuning controlvalues for each pump laser are identified in a look-up table based onthe peak amplification output level from each of the pump lasersdetermined in the simulation. Alternatively, or complementarily, thecontrol unit 206 may be programmed to adjust an amount of attenuationexhibited by programmable attenuators, optically coupled to each of thepump lasers, so as to control the respective optical outputs of the pumplasers, consistent with obtaining the target amplification performance.Thus, the amplifier parameters are associated with an amount of opticalenergy applied to the optical fiber carrying the fiber as well as thewavelength of the optical energy applied, and need not only be thedriving currents applied to the pump lasers.

The transition from step S6 to step S8 in the control process of FIG. 22is a transition from initiating an amplification performance, that ispresumably reasonably close to the target amplification performance, tomonitoring and adjusting the actual amplification performance to bewithin a predetermined tolerance band of the target amplificationperformance. This monitoring and adjusting portion of the controlprocess begins in step S8, where the control unit 206 monitors theoutput WDM signal, and perhaps also monitors the actual input WDMsignal, as discussed above with regard to FIG. 19.

Step S8 may be accomplished in a variety of ways. One way is to takeseveral measurements across the amplification band, such as one meanpower measurement per pump laser. In this scenario, there would be aone-to-one correspondence between the amplifier sub-band and each pumplaser. However, there is no restriction on the granularity with whichthe monitoring step is performed. The greater the resolution (i.e.,number of sample points per Hz), the greater the ability to determinethe degree to which the actual amplification performance matches that ofthe target amplification performance. However, after the resolutionapproaches a level that corresponds with the closest pump laser spacing(e.g., not closer than 6 nm for separate pump sources), little furtherbenefit is achieved unless multiple laser outputs are combined so as toincrease the optical output power. At the other extreme, if theresolution is restricted to only a few points, there is a limitedability to determine whether there are inflection points between samplepoints. Thus, having a resolution that generally corresponds with thepump spacing helps to ensure reliable conformance with the targetamplification performance across the entire amplification band, whilenot wasting processing resources. After obtaining the sample points, thecontrol unit 206 stores the sample points of the output WDM signal, forexample in the main memory 904 for subsequent processing.

After step S8, the process proceeds to step S10, where an inquiry ismade to determine if the actual (monitored) amplification performance iswithin a certain tolerance (γ, e.g., 0.5 dB for strict compliance, or 1dB for less strict compliance) of the target amplification performance,throughout the amplification band. In one embodiment, this determinationis made on a sample-by-sample basis, according to the followingequation:

 ABS[target−monitored]≦γ(a certain tolerance)

Alternatively, an average of the monitored samples may be combined todevelop a mean amplification performance over a predetermined sub-band.In this case, it is possible to reduce the number of calculationsrequired, but also permit the control unit 206 to measure for acompliance of “shape” with regard to the target amplificationperformance. For example, as will be discussed below, the control unit206 may control groups of pump lasers to affect a desired amplificationperformance. Suppose the control of the pump lasers is handled bycontrolling the pump lasers as two groups. The control unit 206 can thencalculate a mean output level for the shorter wavelengths (first group)and another mean output level for the longer wavelengths (second group).This allows the control unit 206 to (1) determine whether the meanamplification performance across the amplification band is within γ; and(2) to determine if an adjustment needs to be made to the slope (i.e.,tilt) of the total amplification performance by some amount.

Now, referring to the flowchart of FIG. 22, if the response to theinquiry in step S10 is affirmative (yes) for all sample points (oralternatively, for a predetermined number of sample points or percentageof all sample points), the process returns to step S8. However, if theresponse to the inquiry in step S10 is negative (no), the processproceeds to step S12.

In step S12, the control unit 206 compares the amount of deviationobserved for each of the sample points. If there is not a consecutivepattern of deviations (e.g., adjacent samples that are both outside thepredetermined tolerance), then the control unit 206 implements anadjustment process that adjusts (up or down) an optical output from thepump laser and/or the central wavelength of the pump laser whose peakamplification profile is most closely aligned with where the deviationoccurred. Moreover, if the control unit observes that the deviation isisolated to a small part of the amplification band, then the controlunit 206 adjusts that optical output and/or central wavelength for thepump laser whose peak output most strongly influences that part of theamplification band.

However, if the control unit 206 determines in step S12 that a series ofadjacent samples of the monitored amplification profile deviate by morethan the predetermined tolerance from the target amplificationperformance, then the control unit 206 implements a different process.In this latter situation, the control unit 206 creates a “secondarytarget amplification profile.” The shape of this secondary targetamplification profile is formed from a difference between the targetamplification performance and the monitored amplification profile. Muchlike with step S4, the control unit 206 then determines (e.g., throughsimulation or table look-up) a set of amplifier parameters (e.g., pumplaser drive currents and/or tunable fiber grating wavelength tuningcontrol values) that can be applied to the pump lasers 202, 203, 204,205 and/or tunable fiber gratings 209, 210, 211, 212 so as to have themonitored amplification profile more closely match the targetamplification profile. Thus, the control unit 206 creates secondarytarget amplification profile that, when added to the monitoredamplification profile, results in a new amplification profile that morereliably falls within the predetermined tolerance of the targetamplification profile, across the amplification band.

Once step S12 is completed, the process proceeds to step S14, where thecontrol unit 206 causes the optical outputs and/or the tunable fibergrating wavelength tuning control values of the affected pump lasers tobe adjusted by an amount necessary to implement the secondary targetamplification profile. The process then returns to step S8 for continuedmonitoring and adjusting operations.

Setting and maintaining an amplification profile may be accomplishedwith the control unit 206 by adjusting individual pump laser outputsand/or the central wavelengths at which the individual pump lasersoperate. However, control can also be accomplished by adjusting theelement profiles of respective groups of pump lasers. As discussedabove, each of the element profiles is realized by combining therespective gain profiles of the pump lasers in that group. Then, theelement gains themselves may be adjusted to lessen the number of degreesof freedom in the control process when making changes to the totalamplification performance. For example, the levels of two elementprofiles may be adjusted quickly to impart a slope change on the totalamplification profile. As discussed above, a positive slope may becreated by increasing the gain of the second element and/or decreasingthe gain of the element profile for the first element. Conversely, anegative slope may be imparted on the total amplification performance ifthe profile for the first element is increased and/or the profile forthe second element is decreased.

A computer-based simulation process, as discussed above, may be used toefficiently determine a magnitude by which each of the element profilesshould be adjusted so as to realize the desired effect. For example, atwo step simulation process may be employed where the element profilesare identified via an element analysis (i.e., performing simulations todetermine the respective gain levels attributable to each of the pumplasers to achieve the desired element profile). Then, a second stepwould be performed where the levels of the element profiles are adjustedto provide a desired total amplification profile of a predeterminedshape. As one example, the element profiles could be developed during aninitial setup mode of operation (e.g., step S4 in FIG. 22) and then thesecond simulation step would be performed in step S12 to identify anamount by which respective element profiles should be adjusted tomaintain the total amplification performance to stay within thepredetermined tolerance.

With regard to initially establishing the element profiles, the presentinventors observed that since a high effective gain is expected with alower power required, an element gain profile for the longer wavelengthband (second element profile) is temporally set based on the desiredtarget amplification gain profile. Moreover, the second element profileis set at a sufficiently high gain to ensure that the second elementprofile substantially accounts for the gain required at the longerwavelength portion of the amplification band. When done this way, theburden is then shifted on the control unit 206 to set the first elementgain profile to match the difference between the target amplificationprofile and the second element gain profile. Since there are more pumplasers in the shorter wavelength group (i.e., the first group), thereare more pump lasers available to create a more complex shaped firstelement profile.

Moreover, since the pump lasers of the present invention are tunable,the central wavelengths of the individual pump lasers, as determined bythe simulation, may also be tuned by the control unit 206. The presentinventors have recognized that by having a tunable Raman amplifier, thatgeneric amplifiers can be installed and tuned after they are installedin order to implement a desired amplification profile. As discussedabove, this field tunability allows for a single module design to beused in a variety of systems, and greatly reduces the manufacturing,inventory, and cost problems associated with custom-built equipment.

When setting the shape for the first element profile, the control unit206 may take into account additional conditions. For example, one of theadditional conditions may be an amount of fiber loss in the opticalfiber. This fiber loss may be determined at the time of installation ofthe Raman amplifier, and thus varies depending on the operationalsetting for that Raman amplifier. The fiber loss may change over time,perhaps based on a system operator adding another amplifier closer tothe subject Raman amplifier, thus decreasing the distance over which theoutput optical signal must travel before being amplified again. As anexample of a further “additional condition”, the first element profilemay be adapted to compensate for pump-to-pump interactions that may beexperienced. FIG. 28 illustrates how pump-to-pump interactions will tendto inflate the total amplification performance at the longer wavelengths(profile “f”). By having the control unit 206 account for theseadditional conditions, it is possible to have the control unit 206 alterthe shape of the respective element profiles, such that the totalamplification performance is optimized, despite the existence of theadditional conditions in which that Raman amplifier will operate.

As another example of how the control unit 206 may adjust the elementprofiles as a simplified control mechanism (as compared withsimultaneously adjusting all of the pump lasers), suppose a tilted gainis observed when a flat gain is targeted. In this situation, the controlunit 206 may correct for the tilted gain by adjusting the optical outputof the first group of pumps and/or the second group of pumps. Oneadjustment process is to make incremental changes to the driving currentfor each pump until the total gain profile becomes readjusted to therelatively flat total gain profile “f” in FIG. 26B. The driving currentincrements are then stored in a memory, for example, the main memory 904for quick retrieval when making adjustments to the total amplificationprofile in future situations. Alternatively, the Raman amplifier mayhave several “hot spare” pumps which can be activated and tuned toprovide amplification at a particular wavelength to remove the tilt.

The control unit 206 may be purposefully configured to impart a tiltedgain by adjusting the levels of the element profiles. For example, aflat gain across the amplification band may be appropriate if thecommunication parameters in the optical communication link are uniformacross the spectral band. However, loss in an optical fiber iswavelength dependent, and thus, some channels of the WDM signal may beattenuated more than others when transiting between cascaded amplifiers.In this case, the control unit 206 may offset this operating conditionby “pre-emphasizing” the optical signals that tend to be attenuated moreby adjusting the element profiles to create a tilted total.

A numeric example will now be provided as a further explanation abouthow a simplified control process implemented in the control unit 206 canuse groups of lasers to set and maintain an amplification gainperformance within a predetermined tolerance of a target amplificationperformance. Assume a power level of respective WDM optical signals,e.g., channels 1-10, is uniform at −20 dBm. Also assume a net targetgain applied by the Raman amplifier is about 10 dB, considering theinternal losses imparted by the tap coupler and the WDM coupler.Consequently, the actual per channel output power level from the Ramanamplifier should be uniform and have a value of about −10 dBm (i.e., −20dBm+10 dB). The control unit 206 may keep as a stored value anindication that the output signal level, per channel is −10 dBm. Thus,if the control unit 206, during its monitoring operation determines thatthe output signal per channel is above or below that −10 dBm signallevel by more than a specified amount, the control unit 206 can increaseor decrease the element profile by an appropriate amount to counteractthe deviation from the expected output level. For example, the controlunit 206 may determine the following actual output power levels andcorresponding deviations from the target value of −10 dBm:

Channel # Actual output power level Deviation from target Channel 1:−15.0 dBm 5.0 dBm Channel 2: −14.5 dBm 4.5 dBm Channel 3: −14.0 dBm 4.0dBm Channel 4: −13.5 dBm 3.5 dBm Channel 5: −13.0 dBm 3.0 dBm Channel 6:−12.5 dBm 2.5 dBm Channel 7: −12.0 dBm 2.0 dBm Channel 8: −11.5 dBm 1.5dBm Channel 9: −11.0 dBm 1.0 dBm Channel 10: −10.5 dBm 0.5 dBm

The control unit 206 will then make the determination that a series(i.e., more than one) deviations exist between sample points and thuswill adjust an element profile up or down in gain level. By observingthe series of deviations, the control unit 206 has in effect determinedthe “secondary target amplification profile.” The control unit may thuscompensate for this deviation by increasing the element gain of thefirst group so as to create a more tilted total amplification profilethat more closely matches the target amplification profile. Furtheradjustments to the first element profile may then be made by adjustingoptical outputs of the pump lasers within the first group, if necessary.

Alternatively, the control unit 206 may refer to the memory to identifydrive current adjustments that have previously been associated with thesecondary target amplification profile that is presently observed.Moreover, the memory holds sets of drive current adjustment values forthe respective pumps in the first and second groups that are prearrangedto create particular secondary target amplification profiles. Since onlya limited number of pre-stored secondary target amplification profilescan be held in memory, the control unit 206 performs a least-squaresanalysis, based on the observed secondary target amplification profileto select a “closest” pre-stored secondary target amplification profile.Other pattern recognition processes may be used as well to selectappropriate pre-stored secondary target amplification profiles, for thepurpose of retrieving the drive current settings, or, alternatively thetunable fiber grating wavelength tuning control value settings,associated with the pre-stored secondary target amplification profiles.

Alternatively, the control unit 206 can be configured to determine anaverage or mean value of the deviations and verify if this average ormean value is within an allowable tolerance. For example, the controlunit 206 may determine that the average value of the deviations is zero,which in some instances may indicate that the amplification performancehas been satisfied. If it is not within tolerance however, the controlunit 206 would increase/decrease the element gains as appropriate tomore closely close the gap between the actual output level profile andthe target level profile (i.e., −10 dBm in this example).

Further, the deviations shown above correspond to a difference betweenthe actual output power level and the target output power level. Note,however, the control unit 206 may determine the drive currents from thesuperposition principle, as discussed above.

Raman amplifiers will be placed in service in a variety of differentoperational conditions that may influence how an optimum target gainprofile is identified for that operational environment. Informationabout the operational conditions (such as the pump-to-pump interactionshown as the difference between profiles “f” and “g” in FIG. 27) isprovided to the control unit 206 at step S2 (FIG. 22) for selecting theoptimum target profile. The source of this information may be foundduring equipment installation, or periodic recalibration. When a targetgain profile “g” is initially given in step S2, yet an actual gainprofile is observed like “f”, this difference in actual from predictedperformance is found in steps S8 and S10. The control unit 206 mayobserve this deviation as being attributable to the existence ofpump-to-pump interaction, that had not originally been considered insteps S2 and S4 when establishing the target amplification performance.Once this observation is made, the process of FIG. 22 may reestablish anew target profile in steps S2 and S4 that consider the existence ofpump-to-pump interaction, rather than just simple superposition. In thiscase, with the modified process for developing the target amplificationperformance, the observed variation from target amplificationperformance to the monitored amplification performance should narrow,thus requiring less adjustment to maintain the predetermined gainprofile.

This additional condition information (which in this case is therealization that there is pump-to-pump interaction) is also consideredin steps S10 and S12 (FIG. 22) when performing the monitoring andanalyzing process steps. The information is useful since the additionalconditions will be reflected in the target amplification profile, or inobserved, consistent deviations from the target amplification profile.The output level or central wavelengths of the pumps in each group maythen be changed as necessary so as to maintain the target amplificationperformance (step S14).

For example, the control unit 206 may incrementally increase or decreasean output power or tune the central wavelength of each pump in the firstgroup so as to effect the element amplification profile of the firstgroup. If the observed total gain profile is still not within theallowable tolerance of the target gain profile after this incrementalincrease or decrease, the control unit 206 may again incrementallyincrease or decrease the output power or tune the central wavelength ofeach group of pumps.

A look-up table, such as that shown in Table 1, may be used to implementthis incremental approach. That is, the control unit 206 may select afirst profile #1 for the set of four pumps in the first group and readthe driving currents from the table for each pump (i.e., a drivingcurrent of 500 mA for each pump). If the actual amplification profiledoes not equal the target amplification profile, the control unit mayselect profile #2 for the four pumps in the first group. Thisincremental approach may be continued until the actual amplificationprofile is within the allowable tolerance of the target amplificationprofile.

TABLE 1 Group 1 (four pumps) Pump 1 Pump 2 Pump 3 Pump 4 Profile Number500 mA 500 mA 500 mA 500 mA #1 490 mA 490 mA 490 mA 490 mA #2 480 mA 480mA 480 mA 480 mA #3

In addition, the look-up Table 1 may also be modified to store differentdrive currents and/or different tunable fiber grating wavelength tuningcontrol values corresponding to different types of amplificationprofiles.

For example, the look-up Table 2 shown below may be used by the controlunit 206 to provide various element gain profiles for a specified groupof pumps. The values in Table 2 were selected as examples to show thatdifferent sets of values may be selected.

TABLE 2 Group 1 (four pumps) Pump 1 Pump 2 Pump 3 Pump 4 Profile Number560 mA 311 mA 122 mA 244 mA #1 560 mA 500 mA 440 mA 330 mA #2 480 mA 480mA 480 mA 480 mA #3

In this example, the control unit 206 may determine that the gainprofile #2 is suitable to offset a fiber loss characteristic (i.e.,another “additional” condition”). The control unit 206 may then read thenecessary driving currents from the table to achieve a desiredamplification profile for the first group of pumps. The control unit 206may select the best profile (i.e., one that minimizes a differencebetween an actual output signal level and a target output signal level,considering the effects of any additional conditions such as fiber loss)from the table based on a variety of factors. For example, the controlunit 206 may select profile #3 that provides the actual amplificationperformance, due to both the first and second groups having a profile 5dB below a target gain profile. The control unit 206 may also determinethis drop in gain occurs in the wavelengths corresponding to the pumpsin the first group. The control unit 206 may then select profile #1 fromTable 2 (which has previously been determined as the best profile tooffset a loss of 5 dB or less, for example). Once profile #1 isselected, the driving currents associated with profile #1 are thenretrieved from memory and applied to the respective pump lasers.

Using the control unit 206 and a special arrangement of the pumpingdevice, the target amplification performance may be changed. In theexamples that follow, the change of target amplification performance isexplained in the context of attempting to provide a same system-levelperformance for an input WDM signal even though a system parameter haschanged. FIGS. 28-46 illustrate different examples of the control unit206 exerting control over the pump lasers so as to produce a targetamplification gain profile that yields a same overall system performancefor an input WDM signal, even though the communication conditions mayhave changed.

FIG. 28 is a schematic of another pumping device 71 according to thepresent invention, which includes “slots” for eight pump lasers 81-88coupled by a Mach-Zehnder interferometer 90. Each of the pumps 81-88 mayhave their central wavelength and driving current set by the controlunit 206. Alternatively, the control unit 206 may simply not applydriving currents to one or more of the pumps 81-88. This alternativeembodiment enables the production of a “generic pumping device” that maybe fully configured/reconfigured after it is placed in a particularoperational situation. In this way, each amplifier need not be customfit to a particular place in a communication network, but rather thegeneric amplifier may be remotely configured by, for example, the remotedevice controller 121 that downloads operational parameters to the Ramanamplifier. In this case, it is possible that at least some fraction ofthe pump lasers will not be used by the control unit 206 to create thetarget amplification performance. Although not shown in FIG. 28, thecontrol unit 206 asserts control over the pump lasers 81-88 by way ofthe bus 120 so as to tune the central wavelengths and set the drivecurrents of the pump lasers 81-88.

In this example, pumps 85 and 87 are turned off, and the total power ofthe pumps included in the first group, tuned by the control unit 206 tooperate at the shortest wavelength side (i.e., the total power due topumps 81, 82, 83 and 84) is greater than a total power due to the pumpsin the second group tuned by the control unit 206 to operate at thelonger wavelength side (i.e., the total power due to pumps 86 and 88).This provides a flat amplification profile since the control unit 206adjusts the levels of the pump lasers in the first group and the secondgroup to form element profiles that result in a flat profile whencombined. In FIG. 28, the pumps in each of the respective groups producethe same output power, but the total output power due to the pumps tunedby the control unit 206 to operate at the longer wavelength side is setto be sufficiently high to maintain the flat gain profile even thoughonly a subset of the pumps is used.

After the element profiles are established, the control unit 206 maymonitor the actual WDM signal and control each operating pump by tuningthe central wavelength and/or setting the drive current so as tomaintain the target amplification gain profile. Alternatively, thecontrol unit 206 may monitor and control the actual amplificationprofile with respect to two groups of pumps.

The amplification bandwidth can be expanded or contracted by tuning thecentral wavelengths and/or changing the contributions from pump laserstuned to operate at the shortest and longest wavelengths in the group ofpump lasers. FIG. 29 illustrates another example in which this can beaccomplished. It should be noted that in FIG. 29, the center frequenciesof the pump lasers are shown, rather than the central wavelengths. Asshown, the center frequency of the first pump 91 is tuned to operate at211 THz (a wavelength of 1420.8 nm) and the center frequency of thefifth pump is tuned to operate at 95 is 207 THz (a wavelength of 1448.3nm). The pumps 91-95 are tuned by the control unit 206 to be spacedapart from each other at an interval of 1 THz and the light output fromthe pumps 91-95 are combined via the WDM combiner 82 to form a shorterwavelength group. This combined light is then combined via a coupler 99with light output from the longer wavelength group that includes a pump96 operating at a frequency of 205 THz (a wavelength of 1462.4 nm),which is spaced apart from the fifth pump 95 by 2 THz.

FIG. 30 illustrates Raman amplification profiles for the pumps 91-96shown in FIG. 29. The curve “A” represents a total amplification profiledue to all of the pumps 91-96, the curve “B” represents a sum of theamplification profiles due to a group of shorter wavelengths of thefirst five pumps 91-95, and the curve “C” represents an amplificationprofile due to the sixth pump 96. The thin lines in FIG. 31 correspondto amplification profiles for each of the first five pumps 91-95. Bymultiplexing the light output from the pumps 91-95 spaced at intervalsof 1 THz, a smooth curve extending rightwardly and downwardly is formed(i.e., curve “B”). In addition, by adding the curve “B” to anamplification profile extending rightwardly and upwardly (in FIG. 30)due to the light output from the sixth pump 96 (i.e., curve “C”), atotal Raman amplification profile is substantially flat as shown by thecurve “A”. Further, as shown by the thin lines in FIG. 30, a projectionof a first amplification curve and a recess of another amplificationcurve mutually cancel each other when the interval is 1 THz.

FIG. 31 is a graph illustrating an enlarged view of the totalamplification curve “A” shown in FIG. 30. As shown, the amplificationbandwidth at 10 dB extends from about 196 THz (a wavelength of 1526.6nm) to about 193 THz (a wavelength of 1553.3 nm) and a gain deviation ofabout 0.1 dB is achieved.

FIG. 32 shows amplification profiles when the center frequency of thepump 96 in FIG. 29 is tuned by the control unit 206 to be spaced apartfrom the fifth pump 95 by 2.5 THz (rather than being spaced apart fromthe fifth pump 95 by 2.0 THz as in FIG. 29). Similar to FIG. 30, thecurve “A” represents the total amplification profile, the curve “B”represents the sum of amplification profiles due to the first five pumps91-95, and the curve “C” represents an amplification profile of thesixth pump 96. Further, the thin lines represent individualamplification profiles of the first five pumps 91-95.

FIG. 33 is an enlarged view of the total amplification curve “A” shownin FIG. 32. As shown, the peak amplification is at 10 dB, theamplification bandwidth extends from about 196 THz (a wavelength of1529.6 nm) to about 192 THz (a wavelength of 1561.4 nm) and anamplification deviation of about 0.1 dB is achieved. Further, theamplification bandwidth is wider than that in FIG. 31, but a largerripple occurs at a middle portion of the bandwidth. The ripple is causedbecause the interval between the fifth pump 95 and the sixth pump 96 islarger (i.e., 2.5 THz rather than 2.0 THz). Thus, in FIG. 33, a largeramplification bandwidth is achieved, but there is a larger ripple at amiddle portion of the bandwidth. The expansion in bandwidth can becontrolled by tuning spare pump lasers to intervals below the centerfrequency of the pump laser that produce the shortest wavelength used todevelop profile “B”, and above a center frequency of longest wavelengthused to develop profile “C” in FIG. 30. While having spare pumpsavailable is more expensive than not including spare pumps, the Ramanamplifier is quickly and easily reconfigured from, for example, a remotedevice controller 121 to adjust an amplification bandwidth.

FIG. 34 is a schematic illustrating yet another Raman amplifier exampleaccording to the present invention. In this example, the frequency ofthe first pump 101 is tuned by the control unit 206 to operate at 211THz (a wavelength of 1420.8 nm) and the frequencies of the second toeighth pumps 102-108 are tuned by the control unit 206 to operate from210 THz (a wavelength of 1427.6 nm) to 204 THz (a wavelength of 1469.6nm). Each of pumps 101-108 is tuned by the control unit 206 to be spacedapart from each other by an interval of 1 THz. Note again, however, thatone or more of the pumps (e.g., pumps 106, 107) may not used (althoughthey may remain in the Raman amplifier to enable for dynamicreconfiguration of the amplification bandwidth, discussed above). Inaddition, the wavelength interval between adjacent operating pumps istuned by the control unit 206 to be within an inclusive range from 6 nmto 35 nm. Further, the number of pumps tuned to operate at the shorterwavelength side (with respect to the middle wavelength between theshortest and longest center wavelengths) is greater than the number ofpumps tuned to operate at the longer wavelength side. That is, the pumpsare tuned such that the central frequency between the first pump 101 andeighth pump 108 is at about 207.5 THz. Thus, pumps 101-104 (i.e., fourpumps) are tuned to operate on the shorter wavelength side and pumps 105and 108 (i.e., two pumps) are tuned to operate on the longer wavelengthside.

FIG. 35 illustrates Raman amplification profiles that are produced whenthe pumps 101-105 and 108 shown in FIG. 34 are used. The curve “A”represents the total amplification profile, the curve “B” represents thesum of amplification profiles due to the first five pumps 101-105, andthe curve “C” represents an amplification profile due to the eighth pump108. In addition, the thin lines represent individual amplificationprofiles of the first five pumps 101-105.

FIG. 36 is an enlarged view of the total amplification curve “A” in FIG.35. As shown, the peak amplification is at 10 dB, the amplificationbandwidth extends from about 196 THz (a wavelength of 1529.6 nm) toabout 191 THz (a wavelength of 1569.6 nm) and the amplificationdeviation is about 0.1 dB. Note the amplification bandwidth is widerthan the amplification bandwidths shown in FIGS. 31 and 33. The reasonis because the eighth pump 108 is tuned to be spaced at a largerinterval (i.e., 3 THz) from the adjacent operating pump 105.

FIG. 37 is a schematic illustrating still another Raman amplificationexample according to the present invention. The frequency of the firstpump 111 is tuned by the control unit 206 to operate at 211 THz (awavelength of 1420.8 nm) and the frequencies of the second to eighthpumps 112 to 118 are tuned by the control unit 206 to operate from 210THz (a wavelength of 1427.6 nm) to 204 THz (a wavelength of 1469.6 nm).In addition, each of the pumps is tuned to be spaced at an interval of 1THz. In this example, the fifth and sixth pumps 115 and 116 are notused. Further, the interval between operating adjacent pumps is tuned tobe within an inclusive range of 6 nm to 35 nm, and the number of pumpstuned to operate on the shorter wavelength side is greater than thenumber of pumps tuned to operate on the longer wavelength side.

FIG. 38 illustrates Raman amplification profiles for the pumps 111-114and 117-118 shown in FIG. 37. The curve “A” represents the totalamplification profile, the curve “B” represents the sum of theamplification profiles due to the first four pumps 111-114, and thecurve “C” represents the sum of the amplification profiles due to theseventh and eighth pumps 117 and 118. The thin lines represent theamplification profiles due to each of the operating pumps 111-114 and117-118.

FIG. 39 is an enlarged view of the total amplification curve “A” in FIG.38. As shown, the peak amplification is at 10 dB, the amplificationbandwidth extends from about 196 THz (a wavelength of 1529.6 nm) toabout 191 THz (a wavelength of 1569.6 nm) and the amplificationdeviation is about 0.1 dB. Further, in this example, note theamplification curve “C” in FIG. 38 is formed from the individualamplification profiles of pumps 117 and 118, whereas the amplificationcurve “C” in FIG. 35 is formed from the single pump 108. In addition,the maximum gain created by the pumps 117 and 118 is about 5 dB (seeFIG. 37), whereas the maximum gain created by the single pump 108 isabout 8 dB. Thus, in FIG. 38, the two pumps 117 and 118 can be driven ata smaller output power compared to driving a single pump.

FIG. 40 is a schematic illustrating still another Raman amplificationexample according to the present invention. In this example, the pumpingdevice includes a set of thirteen pumps 121-133. Each of the pumps istuned by the control unit 206 to be separated by 1 THz and the firstpump 121 is tuned to have a center frequency of 211 THz (a wavelength of1420.8 nm) and the thirteenth pump 133 is tuned to have a centerfrequency of 199 THz (a wavelength of 1506.5 nm). The eleventh andtwelfth pumps 131 and 132 are not used (e.g., the control unit 206 doesnot apply a driving current to the pumps 131 and 132). In addition, theinterval between adjacent operating pumps is tuned to be within aninclusive range of 6 nm to 35 nm, and the number of pumps tuned tooperate on the shorter wavelength side is greater than the number ofpumps tuned to operate on the longer wavelength side.

In FIG. 41, the curve “A” represents the total amplification profile,the curve “B” represents the sum of the amplification profiles due tothe first to tenth pumps, and the curve “C” represents the amplificationprofile of the thirteenth pump. Further, the thin lines represent theindividual amplification profiles of the first to tenth pumps. FIG. 42is an enlarged view of the total amplification curve “A” in FIG. 41. Asshown, the peak amplification is at 10 dB, the amplification bandwidthextends from about 196 THz (a wavelength of 1529.6 nm) to about 186 THz(a wavelength of 1611.8 nm) and the gain deviation is about 0.1 dB.Thus, by tuning additional pumps to operate toward the longerwavelength, the gain profile can be expanded. When target performance ischanged from the one in FIG. 35 to 38, the pump configuration should bechanged from the one in FIG. 34 to the one in FIG. 37. The change inbandwidth, as discussed with regard to the embodiments of FIGS. 28, 29,34, 37 and 40 can be accomplished by the control unit 206 (not shown inthese figures) switching spare pumps into/out-of the pumping circuit andtuning their center frequencies accordingly. The control unit 206 mayimplement the change in bandwidth in response to a command message sentfrom, for example, the remote device controller 121, by way of thenetwork 122.

FIG. 43 illustrates amplification profiles for an example in which thepumps 130 and 131 are not used (rather than the pumps 131 and 132), asdetermined by the control unit 206. In addition, the interval betweenthe adjacent operating pumps is tuned to be within an inclusive range of6 nm to 35 nm, and the number of pumps tuned to operate on the shorterwavelength side is greater than the number of pumps tuned to operate onthe longer wavelength side. In FIG. 43, the curve “A” represents thetotal amplification profile, the curve “B” represents the sum ofamplification profiles due to the first to ninth pumps, and the curve“C” represents the sum of the amplification profiles of the twelfth andthirteenth pumps. The thin lines represent the individual amplificationprofiles of the operating pumps.

FIG. 44 is an enlarged view of the total amplification curve “A” in FIG.43. As shown, the peak amplification is at 10 dB, the amplificationextends from about 196 THz (a wavelength of 1529.6 nm) to about 186 THz(a wavelength of 1611.8 nm) and the amplification deviation is about 0.1dB. Further, as evident from a comparison of the curves “C” in FIGS. 41and 43, two pumps can be driven at a lower output value (as in FIG. 43),rather than by driving a single pump at a higher output power (as inFIG. 41).

FIG. 45 is a flowchart illustrating an operational procedure of thecontrol unit 206 according to the second aspect of the presentinvention. Steps S6, S8, S10, S12 and S14 are the same as that describedin FIG. 22, and accordingly a detailed description of these steps willbe omitted. The difference between the operational procedure shown inFIG. 45 and that shown in FIG. 22 is the control unit 206 changes anexisting amplification profile (step S30) to a new target amplificationperformance with a different amplification bandwidth. For example, anamplification bandwidth may need to be increased so as to accommodateadditional channels (e.g., as the network grows in capacity). In thisinstance, a network engineer may instruct the control unit 206 (e.g.,via the keyboard and mouse, or remotely via the remote device controller121) to increase (or decrease) the amplification bandwidth.

The control unit 206 then determines the parameters to produce the newtarget profile (step S32). For example, as discussed above withreference to the total amplification profiles shown in FIGS. 31, 33, 36,39, 42 and 44, an amplification bandwidth may be increased by tuning thepumps to increase the wavelength separation of the pump having a largestcentral wavelength from the pump having the next largest centralwavelength. That is, the amplification bandwidth in FIG. 31 is producedby the arrangement shown in FIG. 29 (in which the pump 96 is tuned to beseparated by 2 THz from the pump 95), the amplification bandwidth inFIG. 32 is produced by tuning pumps 96 and 95 so as to separate the pump96 from the pump 95 by 2.5 THz, and the amplification bandwidth in FIG.36 is produced by tuning pumps 108 and 105 so as to separate the pump108 (see FIG. 35) from the pump 105 by 3 THz. Thus, the control unit 206may dynamically change the wavelength spacings between pumps by tuningthe center wavelengths of the pumps so as to change an existingamplification profile. For example, assume a pumping device includesseven pumps each tuned to be separated at a wavelength interval of 1THz. Then, according to the second aspect of the present invention, thecontrol unit 206 may only apply driving current to the first throughfifth pumps and the seventh pump. This would result in a similararrangement as that in shown in FIG. 29.

In another example, the control unit 206 may determine a certain pump isnot operating at a required output power, and then turn on or offcertain pumps to offset the failing pump. For example, with reference toFIG. 34, assume the eighth pump 108 is not properly producing a gain of8 dB, but rather is producing a gain of 5 dB. In this instance, thecontrol unit 206 may apply a driving current to seventh pump 107 (whichwas previously turned off) so the pump 107 produces a gain of 5 dB. Notethis example is similar to that shown in FIG. 37, in which two adjacentpumps may be operated to produce a gain of 5 dB each, rather than onepump operating at a gain of 8 dB. That is, the fifth pump 105 may beturned off in order to operate like the pumping device shown in FIG. 37.

Thus, the control unit 206 may be configured to change an existing Ramanamplification profile to have a different amplification bandwidth. Thischange may be initiated via an external command from a network engineer(locally or remotely) or may be requested by the control unit 206itself. That is, as discussed above, the control unit 206 may determinea certain pump is not producing the required gain (i.e., via themonitoring capabilities of the control unit 206) and then change anexisting amplification profile.

Turning now to FIG. 46, which is a schematic for illustrating anoperational procedure according to the another aspect of the presentinvention. In more detail, FIG. 46 illustrates three cascaded Ramanamplifiers 30, 32 and 34, which are remotely controlled by the remotedevice controller 121. In this example, the remote device controller 121may change a total amplification profile in a first Raman amplifier toeffect changes in a next Raman amplifier. For example, the remote devicecontroller 121 may determine a pump (or pumps) in the Raman amplifier 32is not operating. The remote device controller 121 may then increase acorresponding pump output power (or tune the center frequency) operatingin the Raman amplifier 30 to offset the effect caused by the pump whichdoes not operate in the Raman amplifier 32. Note that the remote devicecontroller 121 may also increase a corresponding pump output power (ortune the center frequency) in the Raman amplifier 34 to offset theeffect caused by the pump not operating in the Raman amplifier 32. Thatis, the remote device controller 121 is capable of controlling an entireoperation of a plurality of cascaded Raman amplifiers such that anoverall operation of the network is enhanced.

In addition, the remote device controller 121 may be connected to eachof the Raman amplifiers via an Internet connection (as discussedpreviously). Thus, a network engineer may effectively monitor thenetwork via the remote device controller 121. The remote devicecontroller 121 may include a web site that is accessible from otherlocations as well, via an Internet Browser, such as MICROSOFT EXPLORER.In this case, the operational status of each Raman amplifier 30, 32, and34 may be monitored continuously. Each Raman amplifier 30, 32, and 34may include a built-in reporting mechanism that provides periodic statusmessages to the remote device controller 121. Alternatively, the remotedevice controller can download a Java, ActiveX, or other executable fileto each of the Raman amplifiers 30, 32, and 34, which may then operateto collect status data for automatic report-back to the remote devicecontroller 121. In this way, a network operator may observe thedifferent target amplification profiles being employed in the respectiveRaman amplifiers 30, 32, 34 and take corrective action to help balanceoperations at a system level, to optimize performance at the systemlevel.

The remote device controller 121 and respective Raman amplifiers eachemploy communications interfaces and processing software to enable theuploading and downloading of active content for inspection by networkoperators and technicians located at any of amplifiers or controller121, but also at remote locations via the world wide web. How the worldwide web operates, including communication tools such as web browsersand web pages is discussed at pages 122-166 of Gralla, P., “How TheInternet Works”, Que, 1999, the entire contents of which areincorporated herein by reference. Similarly, the transfer of activecontent between network resources in discussed in Gralla, pages 170-210,the entire contents of which are incorporated herein by reference.

This invention may be conveniently implemented using a conventionalgeneral purpose digital computer or microprocessor programmed accordingto the teachings of the present specification, as will be apparent tothose skilled in the computer art. Appropriate software coding canreadily be prepared by skilled programmers based on the teachings of thepresent disclosure, as will be apparent to those skilled in the softwareart. The invention may also be implemented by the preparation ofapplication specific integrated circuits or by interconnecting anappropriate network of conventional component circuits, as will bereadily apparent to those skilled in the art.

The present invention includes a computer program product which is astorage medium including instructions which can be used to program acomputer to perform a process of the invention. The storage medium caninclude, but is not limited to, an type of disk including floppy disks,optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs,EEPROMs, magnetic or optical cards, or any type of pure softwareinventions (e.g., word processing, accounting, Internet related, etc.)media suitable for storing electronic instructions.

Obviously, additional numerous modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the present invention may be practiced otherwise than as specificallydescribed herein.

What is claimed is:
 1. A tunable multimode pump source for a Ramanamplifier, comprising: a laser module configured to emit light havingmultiple longitudinal modes; an optical fiber aligned to receive thelight from the laser module; a tunable fiber grating coupled to saidoptical fiber and having a predetermined reflectance bandwidth with acenter reflectance wavelength that is controllably tunable over apredetermined tuning range, said predetermined reflectance bandwidthbeing configured to reflect at least a portion of said light back tosaid laser module so as to stabilize a center light wavelength of thelight and restrict a bandwidth of the light to the multiple longitudinalmodes that fall within the predetermined reflectance bandwidth; and atuning mechanism configured to impart a predetermined mechanical strainon said tunable fiber grating in an amount that causes said centerreflectance wavelength to shift from a first wavelength within saidpredetermined tuning range to a second wavelength within saidpredetermined tuning range.
 2. The tunable multimode pump source ofclaim 1, wherein: said grating is formed in said optical fiber.
 3. Thetunable multimode pump source of claim 2, wherein: said grating is afiber Bragg grating.
 4. The tunable multimode pump source of claim 1,wherein: said predetermined reflectance bandwidth being smaller than alasing bandwidth of said laser module.
 5. The tunable multimode pumpsource of claim 4, wherein: said predetermined reflectance bandwidthbeing less than or equal to 2 nm.
 6. The tunable multimode pump sourceof claim 5, wherein: predetermined reflectance bandwidth being in aninclusive range of 0.5 nm to 1 nm.
 7. The tunable multimode pump sourceof claim 1, wherein: the center reflectance wavelength of said tunablefiber grating is continuously tunable over said predetermined tuningrange.
 8. The tunable multimode pump source of claim 1, wherein: saidpredetermined tuning range is at least 60 nm.
 9. The tunable multimodepump source of claim 8, wherein: said predetermined tuning range is atleast 40 nm.
 10. The tunable multimode pump source of claim 9, wherein:said predetermined tuning range is at least 20 nm.
 11. The tunablemultimode pump source of claim 10, wherein: said predetermined tuningrange is at least 10 nm.
 12. The tunable multimode pump source of claim11, wherein: said predetermined tuning range is at least 2 nm.
 13. Thetunable multimode pump source of claim 1, wherein: said tuning mechanismincludes a piezoelectric element, and a lever package mechanicallycoupled to the piezoelectric element; said tunable fiber grating beingmechanically coupled to said lever package such that when apredetermined voltage is applied to said piezoelectric element, thelever package imparts a tensile strain on the tunable fiber grating. 14.The tunable multimode pump source of claim 13, wherein: said tuningmechanism includes a processor-based controller that produces saidpredetermined voltage which causes the center reflectance wavelength toshift from the first wavelength to said second wavelength.
 15. Thetunable multimode pump source of claim 1, wherein: said tuning mechanismincludes a piezoelectric element, and a lever package mechanicallycoupled to the piezoelectric element; and said tunable fiber gratingbeing mechanically coupled to said lever package such that when apredetermined voltage is applied to said piezoelectric element, thelever package imparts a compressive strain on the tunable fiber grating.16. The tunable multimode pump source of claim 15, wherein: said tuningmechanism includes a processor-based controller that produces saidpredetermined voltage which causes the center reflectance wavelength toshift from the first wavelength to said second wavelength.
 17. Thetunable multimode pump source of claim 1, wherein: said tuning mechanismincludes a movable platform to which said tunable fiber grating ismechanically coupled at one portion thereof; and another portion of saidtunable fiber grating being mechanically coupled to another platform,such that when said movable platform is moved a predetermined distancefrom said another platform, a predetermined amount of strain is impartedon said tunable fiber grating.
 18. The tunable multimode pump source ofclaim 17, wherein: when the movable platform is moved in one direction apredetermined compressive strain is placed on said tunable fibergrating; and when the movable platform is moved in another direction apredetermined tensile strain is placed on said tunable fiber grating.19. The tunable multimode pump source of claim 17, wherein: said tuningmechanism includes an electro-mechanical device that converts a controlsignal into a mechanical force that moves said movable platform by thepredetermined distance.
 20. The tunable multimode pump source of claim1, wherein: said tuning mechanism includes a heating mechanismconfigured to change a temperature of said tunable fiber grating by apredetermined amount so as to shift said center reflectance wavelengthfrom said first wavelength to said second wavelength.
 21. The tunablemultimode pump source of claim 1, further comprising: a semiconductoroptical amplifier disposed between the tunable fiber grating and anotherfiber grating, wherein the tunable fiber grating and the another fibergrating are sampled fiber gratings.
 22. A tunable Raman amplifier foramplifying a WDM optical signal in an amplification medium, comprising:a tunable multimode pump source that includes a laser module configuredto emit light having multiple longitudinal modes, an optical fiberaligned to receive the light from the laser module, and a tunable fibergrating coupled to said optical fiber and having a predeterminedreflectance bandwidth with a center reflectance wavelength that iscontrollably tunable over a predetermined tuning range, saidpredetermined reflectance bandwidth being configured to reflect at leasta portion of said light back to said laser module so as to stabilize acenter light wavelength of the light and restrict a bandwidth of thelight to the multiple longitudinal modes that fall within thepredetermined reflectance bandwidth; an optical coupler configured tooptically interconnect the tunable multimode pump source with theamplification medium so as to apply thereto the light with the multiplelongitudinal modes that fall within the predetermined reflectancebandwidth; and a tuning mechanism configured to controllably alter areflectance characteristic of said tunable fiber grating and change saidcenter reflectance wavelength from a first wavelength to a secondwavelength within the predetermined tuning range, wherein said tuningmechanism is configured to impart a predetermined mechanical strain onsaid tunable fiber grating in an amount that causes said centerreflectance wavelength to shift from a first wavelength within saidpredetermined tuning range to a second wavelength within saidpredetermined tuning range.
 23. The Raman amplifier according to claim22, further comprising: a controller having a memory configured to holdcomputer-readable instructions; and a processor configured to executesaid computer-readable instructions and generate a control signal forapplication to said tuning mechanism so as to change the centerreflectance wavelength from the first wavelength to the secondwavelength.
 24. The Raman amplifier according to claim 23, wherein: saidcontroller having an interface configured to receive a controlinstruction from an external device to change an amplification bandwidthof the Raman amplifier; said processor being configured to form thecontrol signal in response to said control instruction so as to causethe tuning mechanism to change the center reflectance wavelength by apredetermined amount so as to change the amplification bandwidth tocorrespond with that specified in the control instruction from theexternal device.
 25. The Raman amplifier according to claim 22, furthercomprising: a tap coupler configured to provide to the processor asampled WDM output signal from the Raman amplifier, wherein theprocessor is configured to determine from said sampled WDM output signalwhen the center reflectance wavelength is not within a predeterminedtolerance of at least one of said first wavelength and said secondwavelength and form the control signal to reset the center reflectancewavelength to said at least one of said first wavelength and said secondwavelength.
 26. The Raman amplifier according to claim 22, furthercomprising: a tap coupler configured to provide to the processor asampled WDM output signal from the Raman amplifier, wherein theprocessor is configured to determine from said sampled WDM output signalwhen a four wave mixing product is present in a signal bandwidth of theWDM optical signal, and when the four wave mixing product is present toprepare the control signal to adjust the center reflectance wavelengthso as to remove the four wave mixing product from the signal bandwidthof the WDM optical signal.
 27. The Raman amplifier according to claim22, further comprising: another laser module configured to emitmulitmode light at the second wavelength, wherein said processor isconfigured to prepare said control signal with an indication to changethe center reflectance wavelength to said second wavelength in responseto receiving an indication that said another laser module has failed.28. The Raman amplifier according to claim 22, further comprising:another tunable multimode pump source, wherein said controller isconfigured to prepare a second control signal so as to control anoptical output power of at least one of said tunable multimode pumpsource and the another tunable multimode pump source so as to adjustablycontrol a predetermined gain profile of said Raman amplifier over apredetermined amplification bandwidth.
 29. The Raman amplifier accordingto claim 28, wherein: said predetermined gain profile has not more than1 dB ripple throughout a signal bandwidth of said WDM optical signal.30. The Raman amplifier of claim 28, wherein: said processor isconfigured to identify said second wavelength and a level of saidoptical output power so as to produce said predetermined gain profile byway of said at least one of simulation and a table lookup procedure. 31.The Raman amplifier of claim 24, wherein: said interface beingconfigured to connect to the Internet for receiving information from atleast one of the remote computer and an adjacent Raman amplifierconnected to said Raman amplifier by way of the amplification medium.32. The Raman amplifier of claim 31, wherein: said computer readableinstructions include at least one of Java and ActiveX instructionsreceived via the Internet.
 33. The Raman amplifier of claim 24, wherein:said processor is configured to receive via said interface anoperational condition regarding an operational status of an adjacentRaman amplifier so as to cause said processor to identify anotherwavelength to tune the tunable fiber grating so as to offset a change inan amplification performance of said adjacent Raman amplifier.
 34. TheRaman amplifier of claim 32, further comprising: another laser moduleoptically coupled to said tunable fiber grating and configured to emitlight having multiple longitudinal modes in another wavelength band thanthat produced by said laser module, wherein the light from the otherlaser module is reflected by said tunable fiber grating when the centerreflectance wavelength is tuned outside of a lasing bandwidth of saidlaser module.
 35. The Raman amplifier of claim 34, wherein: saidprocessor is configured to select light from at least one of said lasermodule and said another laser module to be applied to said tunable fibergrating.
 36. The Raman amplifier of claim 34, further comprising: acontrollable switch that is configured to selectably switch light fromat least one of said laser module and said another laser module to beapplied to said tunable fiber grating.
 37. The Raman amplifier of claim22, wherein: said grating is a Bragg grating formed in said opticalfiber.
 38. The Raman amplifier of claim 22, wherein: the tuningmechanism is configured to continuously tune the center reflectancewavelength of said tunable fiber grating over said predetermined tuningrange.
 39. The Raman amplifier of claim 22, wherein: said predeterminedtuning range is at least 60 nm.
 40. The Raman amplifier of claim 39,wherein: said predetermined tuning range is at least 40 nm.
 41. TheRaman amplifier of claim 40, wherein: said predetermined tuning range isat least 20 nm.
 42. The Raman amplifier of claim 41, wherein: saidpredetermined tuning range is at least 10 nm.
 43. The Raman amplifier ofclaim 42, wherein: said predetermined tuning range is at least 2 nm. 44.The Raman amplifier of claim 23, wherein said tuning mechanism includesa piezoelectric element, and a lever package mechanically coupled to thepiezoelectric element; and said tunable fiber grating being mechanicallycoupled to said lever package such that when a predetermined voltage isapplied to said piezoelectric element, the lever package imparts atensile strain on the tunable fiber grating, which causes the centerreflectance wavelength to shift from the first wavelength to said secondwavelength.
 45. The Raman amplifier of claim 23, wherein said tuningmechanism includes a piezoelectric element, and a lever packagemechanically coupled to the piezoelectric element; and said tunablefiber grating being mechanically coupled to said lever package such thatwhen a predetermined voltage is applied to said piezoelectric element,the lever package imparts a compressive strain on the tunable fibergrating.
 46. The Raman amplifier of claim 45, wherein said tuningmechanism includes a processor-based controller that produces saidpredetermined voltage which causes the center reflectance wavelength toshift from the first wavelength to said second wavelength.
 47. The Ramanamplifier of claim 23, wherein said tuning mechanism includes a movableplatform to which said tunable fiber grating is mechanically coupled atone portion thereof, and another portion of said tunable fiber gratingbeing mechanically coupled to another platform, such that when saidmovable platform is moved a predetermined distance from said anotherplatform, a predetermined amount of strain is imparted on said tunablefiber grating.
 48. The Raman amplifier of claim 47, wherein: when themovable platform is moved in one direction a predetermined compressivestrain is placed on said tunable fiber grating; and when the movableplatform is moved in another direction a predetermined tensile strain isplaced on said tunable fiber grating.
 49. The Raman amplifier of claim47, wherein: said tuning mechanism includes an electro-mechanical devicethat converts a control signal into a mechanical force that moves saidmovable platform by the predetermined distance.
 50. The Raman amplifierof claim 23, wherein: said tuning mechanism includes a heating mechanismconfigured to change a temperature of said tunable fiber grating by apredetermined amount so as to shift said center reflectance wavelengthfrom said first wavelength to said second wavelength.
 51. The Ramanamplifier of claim 22, further comprising: a semiconductor opticalamplifier disposed between the tunable fiber grating and another fibergrating, wherein the tunable fiber grating and the another fiber gratingare sampled fiber gratings.
 52. A reconfigurable Raman amplifier foramplifying a WDM optical signal in an amplification medium, comprising:a plurality of tunable multimode pump sources that each include a lasermodule configured to emit light having multiple longitudinal modes, anoptical fiber aligned to receive the light from the laser module, and atunable fiber grating coupled to said optical fiber and having apredetermined reflectance bandwidth with a center reflectance wavelengththat is controllably tunable over a predetermined tuning range, saidpredetermined reflectance bandwidth being configured to reflect at leasta portion of said light back to said laser module so as to stabilize acenter light wavelength of the light and restrict a bandwidth of thelight to the multiple longitudinal modes that fall within thepredetermined reflectance bandwidth; an optical coupler configured toapply the light from the plurality of tunable multimode pump sources tothe amplification medium; a memory configured to hold thereinpredetermined center wavelength values that describe different centerreflectance wavelengths for said plurality of tunable multimode pumpsources to produce a predetermined gain profile in said amplificationmedium; and a tuning mechanism configured to receive a control signalfor said tuning mechanism to tune said plurality of tunable multimodepump sources to said different center reflectance wavelengths so as toimplement said predetermined gain profile in said amplification medium.53. The reconfigurable Raman amplifier of claim 52, wherein: each ofsaid plurality of tunable multimode pump sources being configured as ageneric tunable multimode pump source that can be tuned to any of saiddifferent center reflectance wavelengths.
 54. The reconfigurable Ramanamplifier of claim 53, wherein: the tuning mechanism is configured toinitially tune the plurality of tunable multimode pump sources to saiddifferent center reflectance wavelengths once said Raman amplifier isinstalled for use in an operational environment.
 55. A method foradjusting an amplification profile of a Raman amplifier having amultimode pump source coupled to a tunable fiber grating, comprisingsteps of: receiving at a tuning mechanism an amplification profilerequirement; retrieving from memory a parameter associated with tuningsaid tunable fiber grating to a predetermined center reflectancewavelength associated with achieving said amplification profilerequirement; and applying a tuning signal to said tuning mechanism so asto change a center reflectance wavelength of said tunable fiber gratingto said predetermined center reflectance wavelength.
 56. The method ofclaim 55, further comprising steps of: comparing said amplificationprofile requirement to an amplification profile exhibited by said Ramanamplifier; and applying said tuning signal to said tuning mechanism whenit is determined in said comparing step that said amplification profileexhibited by said Raman amplifier is within a predetermined tolerance ofsaid amplification profile requirement.
 57. The method of claim 55,further comprising a step of: adjusting an optical output power of saidmultimode pump source so as to achieve said amplification gain profilerequirement.
 58. A controller for adjusting an amplification profile ofa Raman amplifier having a multimode pump source optically coupled to atunable fiber grating, comprising: means for receiving an amplificationprofile requirement from at least one of a remote source and a memory;means for retrieving from memory a parameter associated with tuning atunable fiber grating to a predetermined center reflectance wavelengthassociated with achieving said amplification gain profile requirement;and means for applying a tuning signal to a tuning mechanism and tuningsaid tunable fiber grating so as to change a center reflectancewavelength thereof to said predetermined center reflectance wavelength.59. The controller of claim 58, further comprising: means for comparingsaid amplification profile requirement to an amplification profileexhibited by said tunable Raman amplifier; and means for applying saidtuning signal to said tuning mechanism when it is determined by saidmeans for comparing that said amplification profile exhibited by saidRaman amplifier is within a predetermined tolerance of saidamplification profile requirement.
 60. The controller of claim 58,further comprising: means for adjusting an optical output power of saidmultimode pump source so as to achieve said amplification gain profilerequirement.
 61. An optical communication system comprising: a firstRaman amplifier; a second Raman amplifier; a controller that isconfigured to monitor an amplification profile of said first Ramanamplifier and send a control signal to said second Raman amplifier so asto instruct said second Raman amplifier to alter a shape of anadjustable amplification profile thereof so as to compensate for adetected imperfection in said amplification profile of said first Ramanamplifier; and an optical fiber configured to transport a WDM opticalsignal therethrough, said first Raman amplifier being configured tooptically amplify said WDM optical signal with the first predeterminedamplification profile, wherein said second Raman amplifier beingconfigured to optically amplify said WDM optical signal with theadjustable amplification profile, said second Raman amplifier includinga tunable multimode pump source having a laser module configured to emitlight having multiple longitudinal modes, another optical fiber alignedto receive the light from the laser module, and a tunable fiber gratingcoupled to said optical fiber and having a predetermined reflectancebandwidth with a center reflectance wavelength that is controllablytunable over a predetermined tuning range, said predeterminedreflectance bandwidth being configured to reflect at least a portion ofsaid light back to said laser module so as to stabilize a center lightwavelength of the light and restrict a bandwidth of the light to themultiple longitudinal modes that fall within the predeterminedreflectance bandwidth.